数组和容器很像
但数组的尺寸不能改变
1 数组为什么特殊
数组与容器区别
效率：数组是简单的线性序列，使得访问非常迅速。但数组长度固定，并且在生命周期内不可改变。容器的灵活，需要付出更多开销。
存储类型：没有泛型前，数组可以存储固定类型元素，容器只能存储Object类型。容器一开始也不能存储基本类型，但新增包装类，容器也能存储基本类型。
新增的泛型和包装类 数组和容器在这点的区别已经不明显
数组仅存优点，就是效率
容器比数组功能更多
package arrays;
import java.util.*;
import static net.mindview.util.Print.*;
class BerylliumSphere {//铍球体
private static long counter;
private final long id = counter++;
public String toString() { return "Sphere " + id; }
}
public class ContainerComparison {
public static void main(String[] args) {
BerylliumSphere[] spheres = new BerylliumSphere[10];//新建一个有10个槽的数组，初始化为null
for(int i = 0; i < 5; i++)
spheres[i] = new BerylliumSphere();//初始化第0-4个槽元素
print(Arrays.toString(spheres));//打印全部槽内元素
print(spheres[4]);//打印第5个槽的元素
List<BerylliumSphere> sphereList =
new ArrayList<BerylliumSphere>();// 新建ArrayList
for(int i = 0; i < 5; i++)
sphereList.add(new BerylliumSphere());//ArrayList 加入5个元素
print(sphereList);//打印整个ArrayList
print(sphereList.get(4));//读取第5个元素
int[] integers = { 0, 1, 2, 3, 4, 5 };
print(Arrays.toString(integers));
print(integers[4]);
List<Integer> intList = new ArrayList<Integer>(
Arrays.asList(0, 1, 2, 3, 4, 5));
intList.add(97);
print(intList);
print(intList.get(4));
}
}
/*
[Sphere 0, Sphere 1, Sphere 2, Sphere 3, Sphere 4, null, null, null, null, null]
Sphere 4
[Sphere 5, Sphere 6, Sphere 7, Sphere 8, Sphere 9]
Sphere 9
[0, 1, 2, 3, 4, 5]
4
[0, 1, 2, 3, 4, 5, 97]
4
*/
2 数组是第一级对象
数组标识符是一个引用，它指向堆中创建的一个真实对象
数组没有初始化之前 不能做任何事情
该对象保存指向其他对象的引用，该对象还有一个只读成员length，是能够容纳元素，不是实际保存的元素个数
下标[]是访问数组的唯一方式
| 数组类型 | 存储 | 初始化 |
|---|---|---|
| 对象数组 | 引用 | null |
| 基本类型数组 | 数值 | 数值型 0 字符型char('u0000' 0) 布尔型false |
package arrays;
import java.util.Arrays;
import static net.mindview.util.Print.*;
public class ArrayOptions {
public static void main(String[] args) {
// Arrays of objects:存储对象数组，保存的是引用
BerylliumSphere[] a; // Local uninitialized variable 本地未进行初始化
BerylliumSphere[] b = new BerylliumSphere[5];//创建长度为5的数组
// The references inside the array are 指向的5个对象的引用初始化为空
// automatically initialized to null:
print("b: " + Arrays.toString(b));
BerylliumSphere[] c = new BerylliumSphere[4];
for(int i = 0; i < c.length; i++)
if(c[i] == null) // Can test for null reference
c[i] = new BerylliumSphere();
// Aggregate initialization:"聚集初始化"语法
BerylliumSphere[] d = { new BerylliumSphere(),
new BerylliumSphere(), new BerylliumSphere()
};
// Dynamic aggregate initialization:动态的聚集初始化
a = new BerylliumSphere[]{//初始长度为2
new BerylliumSphere(), new BerylliumSphere(),
};
// (Trailing comma is optional in both cases)这两种情况下，尾随逗号可选
print("a.length = " + a.length);
print("b.length = " + b.length);
print("c.length = " + c.length);
print("d.length = " + d.length);
a = d;
print("a.length = " + a.length);
// Arrays of primitives:基本类型数组，保存基本类型的值
int[] e; // Null reference 空引用
int[] f = new int[5];
// The primitives inside the array are 基本类型数组初始化值为0
// automatically initialized to zero:
print("f: " + Arrays.toString(f));
int[] g = new int[4];
for(int i = 0; i < g.length; i++)
g[i] = i*i;
int[] h = { 11, 47, 93 };
// Compile error: variable e not initialized:
//!print("e.length = " + e.length);
print("f.length = " + f.length);
print("g.length = " + g.length);
print("h.length = " + h.length);
e = h;
print("e.length = " + e.length);
e = new int[]{ 1, 2 };
print("e.length = " + e.length);
}
}
/*
b: [null, null, null, null, null]
a.length = 2
b.length = 5
c.length = 4
d.length = 3
a.length = 3
f: [0, 0, 0, 0, 0]
f.length = 5
g.length = 4
h.length = 3
e.length = 3
e.length = 2
*/
作业1
package arrays.e1;
class BerylliumSphere {//铍球体
private static long counter;
private final long id = counter++;
public String toString() { return "Sphere " + id; }
}
public class E1 {
public static void E1Method(BerylliumSphere[] berylliumSpheres){
System.out.println(berylliumSpheres);
System.out.println();
}
public static void main(String[] args) {
//最常规初始化
BerylliumSphere[] berylliumSphere = new BerylliumSphere[5];
for(int i = 0; i< berylliumSphere.length;i++)
berylliumSphere[i] = new BerylliumSphere();
//聚集初始化
//普通的聚集数组初始化不能奏效，不允许数组初始化,需要加上new BerylliumSphere[]
//E1.E1Method({new BerylliumSphere(),new BerylliumSphere(),});
// 但可以这样
BerylliumSphere[] b = {new BerylliumSphere(),new BerylliumSphere(),};
E1Method(b);
//动态聚集初始化
E1.E1Method(new BerylliumSphere[]{new BerylliumSphere(),new BerylliumSphere(),});
// 这里显的多余
BerylliumSphere[] c = new BerylliumSphere[]{new BerylliumSphere(),new BerylliumSphere(),};
E1Method(c);
}
}
3 返回一个数组
Java中返回String数组
package arrays;
import java.util.*;
public class IceCream {
private static Random rand = new Random(47);
//创建一个String数组
static final String[] FLAVORS = {//“巧克力”、“草莓”、“香草软糖漩涡”、“薄荷片”、
// “摩卡杏仁软糖”、“朗姆酒葡萄干”、“果仁奶油”、“泥饼”
"Chocolate", "Strawberry", "Vanilla Fudge Swirl",
"Mint Chip", "Mocha Almond Fudge", "Rum Raisin",
"Praline Cream", "Mud Pie"
};
//返回一个String数组
public static String[] flavorSet(int n) {
if(n > FLAVORS.length)//如果种类大于数组长度
throw new IllegalArgumentException("Set too big");
String[] results = new String[n];//创建一个要返回的数组
boolean[] picked = new boolean[FLAVORS.length];//创建一个长度和FLAVORS相等的boolean数组，初始化为false，即未添加过此口味
for(int i = 0; i < n; i++) {//遍历result数组，进行初始化
int t; do
t = rand.nextInt(FLAVORS.length);
while(picked[t]);//如果被标记过，则重复生成
results[i] = FLAVORS[t];//初始化result
picked[t] = true;//标记当前下标的String已经被初始化,避免重复
}
return results;
}
public static void main(String[] args) {
for(int i = 0; i < 7; i++)
System.out.println(Arrays.toString(flavorSet(3)));// 随机选择3种味道，选择7组
}
}
/*
[Rum Raisin, Mint Chip, Mocha Almond Fudge]
[Chocolate, Strawberry, Mocha Almond Fudge]
[Strawberry, Mint Chip, Mocha Almond Fudge]
[Rum Raisin, Vanilla Fudge Swirl, Mud Pie]
[Vanilla Fudge Swirl, Chocolate, Mocha Almond Fudge]
[Praline Cream, Strawberry, Mocha Almond Fudge]
[Mocha Almond Fudge, Strawberry, Mint Chip]
*/
作业2
package arrays.e2;
import arrays.BerylliumSphere;
import java.util.Arrays;
public class E2 {
static BerylliumSphere[] createArray(int size) {
if (size < 0 ) return null;
BerylliumSphere[] result = new BerylliumSphere[size];
for (int i = 0; i < size; i++) {
result[i] = new BerylliumSphere();
}
return result;
}
public static void main(String[] args) {
System.out.println(Arrays.toString(createArray(3)));
System.out.println(Arrays.toString(createArray(0))); //0 为[] 负数 NegativeArraySizeException
}
}
/*
[Sphere 0, Sphere 1, Sphere 2]
[]
*/
4 多维数组
基本类型数组
package arrays;
import java.util.Arrays;
public class MultidimensionalPrimitiveArray {
public static void main(String[] args) {
int[][] a = {//2*3数组
{ 1, 2, 3, },
{ 4, 5, 6, },
};
System.out.println(Arrays.deepToString(a));//打印基本类型或者object
}
}
/*
[[1, 2, 3], [4, 5, 6]]
*/
三维数组
package arrays;
import java.util.Arrays;
public class ThreeDWithNew {
public static void main(String[] args) {
// 3-D array with fixed length:
int[][][] a = new int[2][2][4];
System.out.println(Arrays.deepToString(a));// 基本类型数组初始化为0，对象数组初始化为null
}
}
/*
[[[0, 0, 0, 0], [0, 0, 0, 0]], [[0, 0, 0, 0], [0, 0, 0, 0]]]
*/
粗糙数组：构成矩阵的每个向量都可以具有任意长度。
package arrays;
import java.util.*;
public class RaggedArray {
public static void main(String[] args) {
Random rand = new Random(47);
// 3-D array with varied-length vectors:
int[][][] a = new int[rand.nextInt(7)][][];//一维长度
for(int i = 0; i < a.length; i++) {
a[i] = new int[rand.nextInt(5)][];//二维长度
for(int j = 0; j < a[i].length; j++)
a[i][j] = new int[rand.nextInt(5)];//三维长度
}
System.out.println(Arrays.deepToString(a));
System.out.println(a.length);
}
}
/*
[
[],
[[0], [0], [0, 0, 0, 0]],
[[], [0, 0], [0, 0]],
[[0, 0, 0], [0], [0, 0, 0, 0]],
[[0, 0, 0], [0, 0, 0], [0], []],
[[0], [], [0]]
]
*/
非基本类型的对象数组，粗糙数组
package arrays;
import java.util.Arrays;
public class MultidimensionalObjectArrays {
public static void main(String[] args) {
BerylliumSphere[][] spheres = {
{ new BerylliumSphere(), new BerylliumSphere() },
{ new BerylliumSphere(), new BerylliumSphere(),
new BerylliumSphere(), new BerylliumSphere() },
{ new BerylliumSphere(), new BerylliumSphere(),
new BerylliumSphere(), new BerylliumSphere(),
new BerylliumSphere(), new BerylliumSphere(),
new BerylliumSphere(), new BerylliumSphere() },
};
System.out.println(Arrays.deepToString(spheres));
}
}
/*
[
[Sphere 0, Sphere 1],
[Sphere 2, Sphere 3, Sphere 4, Sphere 5],
[Sphere 6, Sphere 7, Sphere 8, Sphere 9, Sphere 10, Sphere 11, Sphere 12, Sphere 13]
]
*/
自动包装机制对数组初始化也起作用
package arrays;
import java.util.Arrays;
public class AutoboxingArrays {
public static void main(String[] args) {
Integer[][] a = { // Autoboxing:
{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 },
{ 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 },
{ 51, 52, 53, 54, 55, 56, 57, 58, 59, 60 },
{ 71, 72, 73, 74, 75, 76, 77, 78, 79, 80 },
};
System.out.println(Arrays.deepToString(a));
}
}
多维数组逐维初始化
package arrays;
import java.util.Arrays;
public class AssemblingMultidimensionalArrays {
public static void main(String[] args) {
Integer[][] a;
a = new Integer[3][];
for(int i = 0; i < a.length; i++) {
a[i] = new Integer[3];
for(int j = 0; j < a[i].length; j++)
a[i][j] = i * j; // Autoboxing
}
System.out.println(Arrays.deepToString(a));
}
}
/*
[[0, 0, 0], [0, 1, 2], [0, 2, 4]]
*/
Integer 和Double数组，自动包装机制，创建包装器对象
package arrays;
import java.util.Arrays;
public class MultiDimWrapperArray {
public static void main(String[] args) {
Integer[][] a1 = { // Autoboxing
{ 1, 2, 3, },
{ 4, 5, 6, },
};
Double[][][] a2 = { // Autoboxing
{ { 1.1, 2.2 }, { 3.3, 4.4 } },
{ { 5.5, 6.6 }, { 7.7, 8.8 } },
{ { 9.9, 1.2 }, { 2.3, 3.4 } },
};
String[][] a3 = {
{ "The", "Quick", "Sly", "Fox" },
{ "Jumped", "Over" },
{ "The", "Lazy", "Brown", "Dog", "and", "friend" },
};
System.out.println("a1: " + Arrays.deepToString(a1));
System.out.println("a2: " + Arrays.deepToString(a2));
System.out.println("a3: " + Arrays.deepToString(a3));
}
}
/*
a1: [[1, 2, 3], [4, 5, 6]]
a2: [[[1.1, 2.2], [3.3, 4.4]], [[5.5, 6.6], [7.7, 8.8]], [[9.9, 1.2], [2.3, 3.4]]]
a3: [[The, Quick, Sly, Fox], [Jumped, Over], [The, Lazy, Brown, Dog, and, friend]]
*/
数组与泛型
不能实例化 具有参数类型 的数组
Peel
擦除 移除 参数类型信息
数组强制 保证类型安全，必须知道持有的确切类型
参数化数组本身的类型
package arrays;
class ClassParameter<T> {//类参数化，<T> 位置
public T[] f(T[] arg) { return arg; }
}
class MethodParameter {//方法参数化，首选
public static <T> T[] f(T[] arg) { return arg; }//静态
}
public class ParameterizedArrayType {
public static void main(String[] args) {
Integer[] ints = { 1, 2, 3, 4, 5 };
Double[] doubles = { 1.1, 2.2, 3.3, 4.4, 5.5 };
Integer[] ints2 =
new ClassParameter<Integer>().f(ints);//声明的地方不可以使用泛型 但赋值可以
Double[] doubles2 =
new ClassParameter<Double>().f(doubles);
ints2 = MethodParameter.f(ints);
doubles2 = MethodParameter.f(doubles);
}
}
尽管不能创建实际的持有泛型的数组对象，
但可以创建非泛型的数组，然后将其转型
package arrays;
import java.util.*;
public class ArrayOfGenerics {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
List<String>[] ls;//创建一个泛型数组的引用
List[] la = new List[10];//创建一个普通数组
ls = (List<String>[])la; // "Unchecked" warning
ls[0] = new ArrayList<String>();
// Compile-time checking produces an error:编译期类型错误
//! ls[1] = new ArrayList<Integer>();
// The problem: List<String> is a subtype of Object
Object[] objects = ls; // So assignment is OK
// Compiles and runs without complaint:
objects[1] = new ArrayList<Integer>();//正常编译
// However, if your needs are straightforward it is
// possible to create an array of generics, albeit
// with an "unchecked" warning:
List<BerylliumSphere>[] spheres =
(List<BerylliumSphere>[])new List[10];
for(int i = 0; i < spheres.length; i++)
spheres[i] = new ArrayList<BerylliumSphere>();
}
}
作业9
package arrays.e9;
import java.util.ArrayList;
class Banana {
private final int id;
Banana(int id) {
this.id = id;
}
public String toString() {
return getClass().getSimpleName() + " " + id;
}
}
class Peel<T> {
T fruit;
Peel(T fruit) {
this.fruit = fruit;
}
void peel() {
System.out.println("Peeling " + fruit);
}
}
public class E9 {
public static void main(String[] args) {
//Peel<Banana> a = new Peel<Banana>[10];// 编译错误 泛型数组创建
ArrayList<Peel<Banana>> a = new ArrayList<>();
for (int i = 0; i < 10; i++) {
a.add(new Peel<>(new Banana(i)));
}
for (Peel<Banana> p : a)
p.peel();
}
}
6 创建测试数据
介绍各种工具填充数组
Arrays.fill
| 分类 | 填充 |
|---|---|
| 基本类型 | 同一个值填充 |
| 对象 | 复制同一个引用 |
package arrays;
import java.util.Arrays;
import static net.mindview.util.Print.*;
public class FillingArrays {
public static void main(String[] args) {
int size = 6;
//创建长度为6的基本类型 和 String类型数组
boolean[] a1 = new boolean[size];
byte[] a2 = new byte[size];
char[] a3 = new char[size];
short[] a4 = new short[size];
int[] a5 = new int[size];
long[] a6 = new long[size];
float[] a7 = new float[size];
double[] a8 = new double[size];
String[] a9 = new String[size];
Arrays.fill(a1, true);
print("a1 = " + Arrays.toString(a1));
Arrays.fill(a2, (byte)11);
print("a2 = " + Arrays.toString(a2));
Arrays.fill(a3,'x');
print("a3 = " + Arrays.toString(a3));
Arrays.fill(a4, (short)17);
print("a4 = " + Arrays.toString(a4));
Arrays.fill(a5, 19);
print("a5 = " + Arrays.toString(a5));
Arrays.fill(a6, 23);
print("a6 = " + Arrays.toString(a6));
Arrays.fill(a7, 29);
print("a7 = " + Arrays.toString(a7));
Arrays.fill(a8, 47);
print("a8 = " + Arrays.toString(a8));
Arrays.fill(a9, "Hello");
print("a9 = " + Arrays.toString(a9));
// Manipulating ranges:
Arrays.fill(a9, 3, 5, "World");//可以指定位置填充,[3,5)
print("a9 = " + Arrays.toString(a9));
}
}
/*
a1 = [true, true, true, true, true, true]
a2 = [11, 11, 11, 11, 11, 11]
a3 = [x, x, x, x, x, x]
a4 = [17, 17, 17, 17, 17, 17]
a5 = [19, 19, 19, 19, 19, 19]
a6 = [23, 23, 23, 23, 23, 23]
a7 = [29.0, 29.0, 29.0, 29.0, 29.0, 29.0]
a8 = [47.0, 47.0, 47.0, 47.0, 47.0, 47.0]
a9 = [Hello, Hello, Hello, Hello, Hello, Hello]
a9 = [Hello, Hello, Hello, World, World, Hello]
*/
数据生成器
package arrays;
import net.mindview.util.Generator;
public class CountingGenerator {
public static class
Boolean implements Generator<java.lang.Boolean> {
private boolean value = false;
public java.lang.Boolean next() {
value = !value; // Just flips back and forth 仅仅取反
return value;
}
}
public static class
Byte implements Generator<java.lang.Byte> {
private byte value = 0;
public java.lang.Byte next() {
return value++;
}
}
static char[] chars = ("abcdefghijklmnopqrstuvwxyz" +
"ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();
public static class
Character implements Generator<java.lang.Character> {
int index = -1;
public java.lang.Character next() {
index = (index + 1) % chars.length;//循环读取下个字符
return chars[index];
}
}
public static class
String implements Generator<java.lang.String> {
private int length = 7;
Generator<java.lang.Character> cg = new Character();//字符生成器
public String() {//无参构造方法
}
public String(int length) {//指定长度构造方法
this.length = length;
}
public java.lang.String next() {
char[] buf = new char[length];
for (int i = 0; i < length; i++)
buf[i] = cg.next();
return new java.lang.String(buf);
}
}
public static class
Short implements Generator<java.lang.Short> {
private short value = 0;
public java.lang.Short next() {
return value++;
}
}
public static class
Integer implements Generator<java.lang.Integer> {
private int value = 0;
public java.lang.Integer next() {
return value++;
}
}
public static class
Long implements Generator<java.lang.Long> {
private long value = 0;
public java.lang.Long next() {
return value++;
}
}
public static class
Float implements Generator<java.lang.Float> {
private float value = 0;
public java.lang.Float next() {
float result = value;
value += 1.0;
return result;
}
}
public static class
Double implements Generator<java.lang.Double> {
private double value = 0.0;
public java.lang.Double next() {
double result = value;
value += 1.0;
return result;
}
}
}
反射
package arrays;
import net.mindview.util.Generator;
public class GeneratorsTest {
public static int size = 10;
public static void test(Class<?> surroundingClass) {
for (Class<?> type : surroundingClass.getClasses()) {
System.out.print(type.getSimpleName() + ": ");
try {
Generator<?> g = (Generator<?>) type.newInstance();//默认构造器
for (int i = 0; i < size; i++)
System.out.printf(g.next() + " ");
System.out.println();
} catch (Exception e) {
throw new RuntimeException(e);
}
}
}
public static void main(String[] args) {
test(CountingGenerator.class);
}
}
/*
Double: 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0
Float: 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0
Long: 0 1 2 3 4 5 6 7 8 9
Integer: 0 1 2 3 4 5 6 7 8 9
Short: 0 1 2 3 4 5 6 7 8 9
String: abcdefg hijklmn opqrstu vwxyzAB CDEFGHI JKLMNOP QRSTUVW XYZabcd efghijk lmnopqr
Character: a b c d e f g h i j
Byte: 0 1 2 3 4 5 6 7 8 9
Boolean: true false true false true false true false true false
*/
随机数生成器
package arrays;
import net.mindview.util.Generator;
import java.util.*;
public class RandomGenerator {
private static Random r = new Random(47);
public static class
Boolean implements Generator<java.lang.Boolean> {
public java.lang.Boolean next() {
return r.nextBoolean();
}
}
public static class
Byte implements Generator<java.lang.Byte> {
public java.lang.Byte next() {
return (byte)r.nextInt();
}
}
public static class
Character implements Generator<java.lang.Character> {
public java.lang.Character next() {
return CountingGenerator.chars[
r.nextInt(CountingGenerator.chars.length)];
} }
public static class
String extends CountingGenerator.String {
// Plug in the random Character generator:
{ cg = new Character(); } // Instance initializer
public String() {}
public String(int length) { super(length); }
}
public static class
Short implements Generator<java.lang.Short> {
public java.lang.Short next() {
return (short)r.nextInt();
}
}
public static class
Integer implements Generator<java.lang.Integer> {
private int mod = 10000;
public Integer() {}
public Integer(int modulo) { mod = modulo; }
public java.lang.Integer next() {
return r.nextInt(mod);
}
}
public static class
Long implements Generator<java.lang.Long> {
private int mod = 10000;
public Long() {}
public Long(int modulo) { mod = modulo; }
public java.lang.Long next() {
return new java.lang.Long(r.nextInt(mod));
}
}
public static class
Float implements Generator<java.lang.Float> {
public java.lang.Float next() {
// Trim all but the first two decimal places:
int trimmed = Math.round(r.nextFloat() * 100);
return ((float)trimmed) / 100;
} }
public static class
Double implements Generator<java.lang.Double> {
public java.lang.Double next() {
long trimmed = Math.round(r.nextDouble() * 100);
return ((double)trimmed) / 100;
} }
}
测试RandomGenerator
package arrays;
public class RandomGeneratorsTest {
public static void main(String[] args) {
GeneratorsTest.test(RandomGenerator.class);
}
}
从Generator中创建数组
package arrays;
import net.mindview.util.CollectionData;
import net.mindview.util.Generator;
public class Generated {
// Fill an existing array:
public static <T> T[] array(T[] a, Generator<T> gen) {//
return new CollectionData<T>(gen, a.length).toArray(a);
}
// Create a new array: 反射
@SuppressWarnings("unchecked")
public static <T> T[] array(Class<T> type,
Generator<T> gen, int size) {
T[] a =
(T[])java.lang.reflect.Array.newInstance(type, size);
return new CollectionData<T>(gen, size).toArray(a);
}
}
ArrayList.toArray(a)
| a.length < size | 返回 (T[]) Arrays.copyOf(elementData, size, a.getClass()); |
|---|---|
| = | 返回a |
|返回a 且a[size]=null|
package arrays;
import java.util.Arrays;
public class TestGenerated {
public static void main(String[] args) {
Integer[] a = { 9, 8, 7, 6 };
System.out.println(Arrays.toString(a));
a = Generated.array(a,new CountingGenerator.Integer());//覆盖 but leaves the original array in place 书上翻译欠妥 保留原来的数组
System.out.println(Arrays.toString(a));
Integer[] b = Generated.array(Integer.class,
new CountingGenerator.Integer(), 15);
System.out.println(Arrays.toString(b));
}
}
/*
[9, 8, 7, 6]
[0, 1, 2, 3]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
*/
泛型不能用于基本类型，解决用生成器填充基本类型数组
转换器：接收任意的包装器对象数组，将其转换为相应的基本类型数据
package arrays;
public class ConvertTo {
public static boolean[] primitive(Boolean[] in) {
boolean[] result = new boolean[in.length];
for(int i = 0; i < in.length; i++)
result[i] = in[i]; // Autounboxing
return result;
}
public static char[] primitive(Character[] in) {
char[] result = new char[in.length];
for(int i = 0; i < in.length; i++)
result[i] = in[i];
return result;
}
public static byte[] primitive(Byte[] in) {
byte[] result = new byte[in.length];
for(int i = 0; i < in.length; i++)
result[i] = in[i];
return result;
}
public static short[] primitive(Short[] in) {
short[] result = new short[in.length];
for(int i = 0; i < in.length; i++)
result[i] = in[i];
return result;
}
public static int[] primitive(Integer[] in) {
int[] result = new int[in.length];
for(int i = 0; i < in.length; i++)
result[i] = in[i];
return result;
}
public static long[] primitive(Long[] in) {
long[] result = new long[in.length];
for(int i = 0; i < in.length; i++)
result[i] = in[i];
return result;
}
public static float[] primitive(Float[] in) {
float[] result = new float[in.length];
for(int i = 0; i < in.length; i++)
result[i] = in[i];
return result;
}
public static double[] primitive(Double[] in) {
double[] result = new double[in.length];
for(int i = 0; i < in.length; i++)
result[i] = in[i];
return result;
}
}
package arrays;
import java.util.Arrays;
public class PrimitiveConversionDemonstration {
public static void main(String[] args) {
Integer[] a = Generated.array(Integer.class,
new CountingGenerator.Integer(), 15);
int[] b = ConvertTo.primitive(a);
System.out.println(Arrays.toString(b));
boolean[] c = ConvertTo.primitive(
Generated.array(Boolean.class,
new CountingGenerator.Boolean(), 7));
System.out.println(Arrays.toString(c));
}
}
/*
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
[true, false, true, false, true, false, true]
*/
package arrays;
import java.util.Arrays;
import static net.mindview.util.Print.print;
public class TestArrayGeneration {
public static void main(String[] args) {
int size = 6;
boolean[] a1 = ConvertTo.primitive(Generated.array(
Boolean.class, new RandomGenerator.Boolean(), size));
print("a1 = " + Arrays.toString(a1));
byte[] a2 = ConvertTo.primitive(Generated.array(
Byte.class, new RandomGenerator.Byte(), size));
print("a2 = " + Arrays.toString(a2));
char[] a3 = ConvertTo.primitive(Generated.array(
Character.class,
new RandomGenerator.Character(), size));
print("a3 = " + Arrays.toString(a3));
short[] a4 = ConvertTo.primitive(Generated.array(
Short.class, new RandomGenerator.Short(), size));
print("a4 = " + Arrays.toString(a4));
int[] a5 = ConvertTo.primitive(Generated.array(
Integer.class, new RandomGenerator.Integer(), size));
print("a5 = " + Arrays.toString(a5));
long[] a6 = ConvertTo.primitive(Generated.array(
Long.class, new RandomGenerator.Long(), size));
print("a6 = " + Arrays.toString(a6));
float[] a7 = ConvertTo.primitive(Generated.array(
Float.class, new RandomGenerator.Float(), size));
print("a7 = " + Arrays.toString(a7));
double[] a8 = ConvertTo.primitive(Generated.array(
Double.class, new RandomGenerator.Double(), size));
print("a8 = " + Arrays.toString(a8));
}
}
/*
a1 = [true, false, true, false, false, true]
a2 = [104, -79, -76, 126, 33, -64]
a3 = [Z, n, T, c, Q, r]
a4 = [-13408, 22612, 15401, 15161, -28466, -12603]
a5 = [7704, 7383, 7706, 575, 8410, 6342]
a6 = [7674, 8804, 8950, 7826, 4322, 896]
a7 = [0.01, 0.2, 0.4, 0.79, 0.27, 0.45]
a8 = [0.16, 0.87, 0.7, 0.66, 0.87, 0.59]
*/
作业1 自动包装机制不能应用于数组
package arrays.e11;
public class E11 {
public static void main(String[] args) {
int[] pa = { 1, 2, 3, 4, 5 };
// Integer[] wa = pa;//编译错误：类型不匹配
Integer[] wb = { 1, 2, 3, 4, 5 };
// int[] pb = wb;//编译错误：类型不匹配
}
}
作业12
package arrays.e12;
import arrays.ConvertTo;
import arrays.CountingGenerator;
import arrays.Generated;
import java.util.Arrays;
public class E12 {
public static void main(String[] args) {
double[] doubles = ConvertTo.primitive(
Generated.array(Double.class,new CountingGenerator.Double(),15));
System.out.println(Arrays.toString(doubles));
}
}
作业14
package arrays.e14;
import arrays.CountingGenerator;
import arrays.RandomGenerator;
import net.mindview.util.Generator;
import java.lang.reflect.Array;
import java.util.Arrays;
class Fill {
public static void primitive(Object array, Generator<?> gen) {
int size = Array.getLength(array);
Class<?> type1 = array.getClass();// "class [Z"
Class<?> type = array.getClass().getComponentType();
//getComponentType ：返回代 数组的组件类型的class。如果这个类不代表一个数组类，返回null
//getClass()方法用来返回对象的类型信息（也就是在内存堆区的那一份唯一的字节码）
for (int i = 0; i < size; i++)
if (type == Boolean.TYPE)
Array.setBoolean(array, i, (Boolean) gen.next());
else if (type == Byte.TYPE)
Array.setByte(array, i, (Byte) gen.next());
else if (type == Short.TYPE)
Array.setShort(array, i, (Short) gen.next());
else if (type == Integer.TYPE)
Array.setInt(array, i, (Integer) gen.next());
else if (type == Character.TYPE)
Array.setChar(array, i, (Character) gen.next());
else if (type == Float.TYPE)
Array.setFloat(array, i, (Float) gen.next());
else if (type == Double.TYPE)
Array.setDouble(array, i, (Double) gen.next());
else if (type == Long.TYPE)
Array.setLong(array, i, (Long) gen.next());
}
}
public class E14 {
public static void main(String[] args) {
int size = 6;
//first create all primitive arrays
boolean[] a1 = new boolean[size];
byte[] a2 = new byte[size];
char[] a3 = new char[size];
short[] a4 = new short[size];
int[] a5 = new int[size];
long[] a6 = new long[size];
float[] a7 = new float[size];
double[] a8 = new double[size];
//now fill them with a matching generator;
Fill.primitive(a1,new CountingGenerator.Boolean());
System.out.println("a1 = " + Arrays.toString(a1));
Fill.primitive(a2,new CountingGenerator.Byte());
System.out.println("a2 = " + Arrays.toString(a2));
Fill.primitive(a3,new CountingGenerator.Character());
System.out.println("a3= " + Arrays.toString(a3));
Fill.primitive(a4,new CountingGenerator.Short());
System.out.println("a4 = " + Arrays.toString(a4));
Fill.primitive(a5,new CountingGenerator.Integer());
System.out.println("a5 = " + Arrays.toString(a5));
Fill.primitive(a6,new CountingGenerator.Long());
System.out.println("a6 = " + Arrays.toString(a6));
Fill.primitive(a7,new CountingGenerator.Float());
System.out.println("a7 = " + Arrays.toString(a7));
Fill.primitive(a8,new CountingGenerator.Double());
System.out.println("a8 = " + Arrays.toString(a8));
}
}
7 Arrays实用功能
Arrays有static方法，其中6个基本方法
equals用于比较数组是否相等（deep Equals用于多维数组）
fill为数组填充单一元素
sort对数组排序
binarySearch在已经排序的数据中查找元素
toString产生数组的String表示
hashCode返回数组的散列码
另：Arrays.asList接受任意序列或数组 返回一个List
7.1 复制数组 System.arraycopy( )
package arrays;
import java.util.Arrays;
import static net.mindview.util.Print.print;
public class CopyingArrays {
public static void main(String[] args) {
int[] i = new int[7];
int[] j = new int[10];
Arrays.fill(i, 47);
Arrays.fill(j, 99);
print("i = " + Arrays.toString(i));
print("j = " + Arrays.toString(j));
System.arraycopy(i, 0, j, 0, i.length);
print("j = " + Arrays.toString(j));
int[] k = new int[5];
Arrays.fill(k, 103);
System.arraycopy(i, 0, k, 0, k.length);
print("k = " + Arrays.toString(k));
Arrays.fill(k, 103);
System.arraycopy(k, 0, i, 0, k.length);
print("i = " + Arrays.toString(i));
// Objects:
Integer[] u = new Integer[10];
Integer[] v = new Integer[5];
// int[] v = new int[5];// 下面的 System.arraycopy 不会执行自动包装和自动拆包，会ArrayStoreException
Arrays.fill(u, 47);
Arrays.fill(v, 99);
print("u = " + Arrays.toString(u));
print("v = " + Arrays.toString(v));
System.arraycopy(v, 0, u, u.length/2, v.length);
print("u = " + Arrays.toString(u));
}
}
/*
i = [47, 47, 47, 47, 47, 47, 47]
j = [99, 99, 99, 99, 99, 99, 99, 99, 99, 99]
j = [47, 47, 47, 47, 47, 47, 47, 99, 99, 99]
k = [47, 47, 47, 47, 47]
i = [103, 103, 103, 103, 103, 47, 47]
u = [47, 47, 47, 47, 47, 47, 47, 47, 47, 47]
v = [99, 99, 99, 99, 99]
u = [47, 47, 47, 47, 47, 99, 99, 99, 99, 99]
*/
作业18 浅复制（复制对象的引用，而不是对象本身的拷贝）
package arrays.e18;
import arrays.BerylliumSphere;
import java.lang.reflect.Field;
import java.util.Arrays;
public class E18 {
static void setID(BerylliumSphere bs,long value){
try {
Field fid = BerylliumSphere.class.getDeclaredField("id");//反射修改私有字段
fid.setAccessible(true);
fid.setLong(bs,value);
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
BerylliumSphere[] a = new BerylliumSphere[4];
Arrays.fill(a,new BerylliumSphere());
BerylliumSphere[] b = new BerylliumSphere[4];
System.out.println("a= " + Arrays.toString(a));
System.out.println("b= " + Arrays.toString(b));
System.arraycopy(a,0,b,0,b.length);
System.out.println("b= " + Arrays.toString(b));
//Changing a reference in 'a' will not impact 'b'
a[1] = a[3] = new BerylliumSphere();
System.out.println("a= " + Arrays.toString(a));
System.out.println("b= " + Arrays.toString(b));
// Changing an object's state will impact 'b', as well.
setID(a[0], -1L);
System.out.println("a = " + Arrays.toString(a));
System.out.println("b = " + Arrays.toString(b));
}
}
/*
a= [Sphere 0, Sphere 0, Sphere 0, Sphere 0]
b= [null, null, null, null]
b= [Sphere 0, Sphere 0, Sphere 0, Sphere 0]
a= [Sphere 0, Sphere 1, Sphere 0, Sphere 1]
b= [Sphere 0, Sphere 0, Sphere 0, Sphere 0]
a = [Sphere -1, Sphere 1, Sphere -1, Sphere 1]
b = [Sphere -1, Sphere -1, Sphere -1, Sphere -1]
*/
7.2 数组的比较
package arrays;
import java.util.Arrays;
import static net.mindview.util.Print.print;
public class ComparingArrays {
public static void main(String[] args) {
int[] a1 = new int[10];
int[] a2 = new int[10];
Arrays.fill(a1, 47);
Arrays.fill(a2, 47);
print(Arrays.equals(a1, a2));
a2[3] = 11;
print(Arrays.equals(a1, a2));
String[] s1 = new String[4];
Arrays.fill(s1, "Hi");
String[] s2 = { new String("Hi"), new String("Hi"),
new String("Hi"), new String("Hi") };
print(Arrays.equals(s1, s2));
}
}
/*
true
false
true
*/
所有基本类型和Object 的 equals方法
/**
* Returns <tt>true</tt> if the two specified arrays of ints are
* <i>equal</i> to one another. Two arrays are considered equal if both
* arrays contain the same number of elements, and all corresponding pairs
* of elements in the two arrays are equal. In other words, two arrays
* are equal if they contain the same elements in the same order. Also,
* two array references are considered equal if both are <tt>null</tt>.<p>
*
* @param a one array to be tested for equality
* @param a2 the other array to be tested for equality
* @return <tt>true</tt> if the two arrays are equal
*/
public static boolean equals(int[] a, int[] a2) {
if (a==a2)//如果数组标识符在堆中指向的对象地址一样，返回true,如果都是null，也是true
return true;
if (a==null || a2==null)//如果仅有一个null,则返回false
return false;
int length = a.length;
if (a2.length != length)//如果两个数组长度不等，返回false
return false;
for (int i=0; i<length; i++)
// 对象
// Object o1 = a[i];
// Object o2 = a2[i];
// if (!(o1==null ? o2==null : o1.equals(o2)))//如果o1为null,判断o2若也为null（或 如果o1不为null，判断Object o1和o2相相等）继续判断下个位置的对象；否则返回不相等
// return false;
//其他类型
if (a[i] != a2[i])
//处理方式不一样的基本类型 ，需要下次看，Double和Float的比较为什么不一样。
// if (Double.doubleToLongBits(a[i])!=Double.doubleToLongBits(a2[i]))
// if (Float.floatToIntBits(a[i])!=Float.floatToIntBits(a2[i]))
return false;
return true;
}
作业19
package arrays.e19;
import java.util.Arrays;
class DataHolder{
protected int data;
DataHolder(int data){this.data=data;}
}
class DataHolderWithEquals extends DataHolder{
DataHolderWithEquals(int data) {
super(data);
}
public boolean equals(Object o){
// 重载方法，
// 判断类型一样，且int 值一样
return o instanceof DataHolderWithEquals && data == ((DataHolderWithEquals)o).data;
}
}
public class E19 {
public static void main(String[] args) {
DataHolder[] a1 = new DataHolder[5];
DataHolder[] a2 = new DataHolder[5];
Arrays.fill(a1,new DataHolder(1));
Arrays.fill(a1,new DataHolder(2));
System.out.println(Arrays.equals(a1,a2));
Arrays.fill(a1,new DataHolderWithEquals(1));
Arrays.fill(a2,new DataHolderWithEquals(1));
System.out.println(Arrays.equals(a1,a2));
}
}
作业20 多维数组的deepEquals()
package arrays.e20;
import java.util.Arrays;
public class E20 {
public static void main(String[] args) {
int[][] table1 = {
{1, 2, 3},
{4, 5},
{6, 7, 8, 9}
};
int[][] table2 = {
{1, 2, 3},
{4, 5},
{6, 7, 8, 9}
};
Integer[][] table3 = {
{1, 2, 3},
{4, 5},
{6, 7, 8, 9}
};
int[][] table4 = {
{1, 2, 3},
{6, 4, 5},
{7, 8}
};
System.out.println(Arrays.deepEquals(table1, table2));
System.out.println(Arrays.deepEquals(table1, table3));
System.out.println(Arrays.deepEquals(table1, table4));
//手动检查table1 和talbe3
boolean res = true;
exit_loop://标签，类似goto
for (int i = 0; i < table1.length; i++)
for (int j = 0; j < table1[i].length; j++)
if (table1[i][j] != table3[i][j]) {
res = false;
break exit_loop; //标签，类似goto
}
System.out.println(res);
}
}
/*
true
false
false
true
*/
7.3 数组元素的比较
编程设计的一个主要目标是“将易变的元素与稳定的元素分开”，在这里，保持不变的代码是一般的排序算法，但是变化的是对象的比较方式。
使用策略设计模式而不是将比较代码放入许多不同的排序源码中。使用策略模式时，变化的代码部分被封装在一个单独的类(策略对象)中。
java有两种方式提供比较功能
数组中的元素实现Comparable接口 即可拥有比较功能
package arrays;
import net.mindview.util.Generator;
import java.util.Arrays;
import java.util.Random;
public class CompType implements Comparable<CompType> {
int i;
int j;
private static int count = 1;
public CompType(int n1,int n2){
i = n1;
j = n2;
}
public String toString(){
String result = "[i = " + i + ", j = " + j + "]";
if (count++ %3 == 0)
result += "
";
return result;
}
@Override
public int compareTo(CompType rv) {//只比较i的值
return (i < rv.i ? -1:(i == rv.i ? 0 :1));
}
private static Random r = new Random(47);
public static Generator<CompType> generator(){
return new Generator<CompType>() {
@Override
public CompType next() {
return new CompType(r.nextInt(100),r.nextInt(100));
}
};
}
public static void main(String[] args) {
CompType[] a = Generated.array(new CompType[12],generator());
System.out.println("before sorting");
System.out.println(Arrays.toString(a));
// Arrays.sort方法内部会将参数直接转换成Comparable对象进行比较
// 如果没有实现Comparable接口 则会报类型转换错误
Arrays.sort(a);
System.out.println("after sorting");
System.out.println(Arrays.toString(a));
}
}
/*
before sorting
[[i = 58, j = 55], [i = 93, j = 61], [i = 61, j = 29]
, [i = 68, j = 0], [i = 22, j = 7], [i = 88, j = 28]
, [i = 51, j = 89], [i = 9, j = 78], [i = 98, j = 61]
, [i = 20, j = 58], [i = 16, j = 40], [i = 11, j = 22]
]
after sorting
[[i = 9, j = 78], [i = 11, j = 22], [i = 16, j = 40]
, [i = 20, j = 58], [i = 22, j = 7], [i = 51, j = 89]
, [i = 58, j = 55], [i = 61, j = 29], [i = 68, j = 0]
, [i = 88, j = 28], [i = 93, j = 61], [i = 98, j = 61]
]
*/
反序排序
package arrays;
import java.util.Arrays;
import java.util.Collections;
public class Reverse {
public static void main(String[] args) {
CompType[] a = Generated.array(new CompType[12],CompType.generator());
System.out.println("before sorting: ");
System.out.println(Arrays.toString(a));
Arrays.sort(a, Collections.reverseOrder());
System.out.println("after sorting: ");
System.out.println(Arrays.toString(a));
}
}
/*
before sorting:
[[i = 58, j = 55], [i = 93, j = 61], [i = 61, j = 29]
, [i = 68, j = 0], [i = 22, j = 7], [i = 88, j = 28]
, [i = 51, j = 89], [i = 9, j = 78], [i = 98, j = 61]
, [i = 20, j = 58], [i = 16, j = 40], [i = 11, j = 22]
]
after sorting:
[[i = 98, j = 61], [i = 93, j = 61], [i = 88, j = 28]
, [i = 68, j = 0], [i = 61, j = 29], [i = 58, j = 55]
, [i = 51, j = 89], [i = 22, j = 7], [i = 20, j = 58]
, [i = 16, j = 40], [i = 11, j = 22], [i = 9, j = 78]
]
*/
策略模式：实现Comparator进行不同的排序策略
package arrays;
import java.util.Arrays;
import java.util.Comparator;
class CompTypeComparator implements Comparator<CompType>{
@Override
public int compare(CompType o1, CompType o2) {
return (Integer.compare(o1.j, o2.j));
// return (o1.j< o2.j ? -1:(o1.j == o2.j? 0:1));
}
}
public class ComparatorTest {
public static void main(String[] args) {
CompType[] a = Generated.array(new CompType[12],CompType.generator());
System.out.println("before sorting");
System.out.println(Arrays.toString(a));
Arrays.sort(a,new CompTypeComparator());
System.out.println("after sorting");
System.out.println(Arrays.toString(a));
}
}
/*
before sorting
[[i = 58, j = 55], [i = 93, j = 61], [i = 61, j = 29]
, [i = 68, j = 0], [i = 22, j = 7], [i = 88, j = 28]
, [i = 51, j = 89], [i = 9, j = 78], [i = 98, j = 61]
, [i = 20, j = 58], [i = 16, j = 40], [i = 11, j = 22]
]
after sorting
[[i = 68, j = 0], [i = 22, j = 7], [i = 11, j = 22]
, [i = 88, j = 28], [i = 61, j = 29], [i = 16, j = 40]
, [i = 58, j = 55], [i = 20, j = 58], [i = 93, j = 61]
, [i = 98, j = 61], [i = 9, j = 78], [i = 51, j = 89]
]
*/
作业21
这个例子很不错！
package arrays.e21;
import arrays.BerylliumSphere;
import arrays.Generated;
import arrays.e15.BSGenerator;
import java.lang.reflect.Field;
import java.util.*;
class ComparableBerylliumSphere extends BerylliumSphere implements Comparable<BerylliumSphere>{
static long getID(BerylliumSphere bs){
try {
Field fid = BerylliumSphere.class.getDeclaredField("id");
fid.setAccessible(true);
return fid.getLong(bs);
} catch (Exception e) {
e.printStackTrace();
return 0L;// Bogus value 虚假的值
}
}
@Override
public int compareTo(BerylliumSphere o) {
long id = getID(this);
long oid = getID(o);
return (Long.compare(id, oid));
}
}
public class E21 {
static void setID(BerylliumSphere bs,long value){
try {
Field fid = BerylliumSphere.class.getDeclaredField("id");
fid.setAccessible(true);
fid.setLong(bs,value);
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
Random rand = new Random(47);
BerylliumSphere[] a = Generated.array(BerylliumSphere.class,new BSGenerator(),5);
System.out.println("Before shuffle: a = " + Arrays.toString(a));
Collections.shuffle(Arrays.asList(a),rand);
System.out.println("Before sort 1: a = " + Arrays.toString(a));
try {
Arrays.sort(a);
System.out.println("After sort 1: a = " + Arrays.toString(a));
}catch (ClassCastException e){
System.out.println("Array cannot be sorted!");
}
System.out.println("***********");
for (int i = 0;i< a.length;i++)
a[i] = new ComparableBerylliumSphere();
System.out.println("Before shuffle: a = " + Arrays.toString(a));
Collections.shuffle(Arrays.asList(a),rand);
System.out.println("Before sort 2: a = " + Arrays.toString(a));
Arrays.sort(a);
System.out.println("After sort before rev 2: a = " + Arrays.toString(a));
Arrays.sort(a,Collections.reverseOrder());
System.out.println("After rev 2: a = " + Arrays.toString(a));
}
}
/*
Before shuffle: a = [Sphere 0, Sphere 1, Sphere 2, Sphere 3, Sphere 4]
Before sort 1: a = [Sphere 2, Sphere 0, Sphere 4, Sphere 1, Sphere 3]
Array cannot be sorted!
***********
Before shuffle: a = [Sphere 5, Sphere 6, Sphere 7, Sphere 8, Sphere 9]
Before sort 2: a = [Sphere 8, Sphere 5, Sphere 9, Sphere 7, Sphere 6]
After sort before rev 2: a = [Sphere 5, Sphere 6, Sphere 7, Sphere 8, Sphere 9]
After rev 2: a = [Sphere 9, Sphere 8, Sphere 7, Sphere 6, Sphere 5]
*/
7.4 数组排序
如果数组元素是基本数据类型或 实现Comparable接口 或Comparator 的对象类型 可以使用内置排序方法直接排序
针对基本类型 源码 可以看到使用的是二分排序
package arrays;
import java.util.Arrays;
import java.util.Collections;
public class StringSorting {
public static void main(String[] args) {
String[] sa = Generated.array(new String[20],new RandomGenerator.String(5));
System.out.println("Before sort: " + Arrays.toString(sa));
Arrays.sort(sa);// 大写前面，小写后面 字典编排顺序排序
System.out.println("After sort: " + Arrays.toString(sa));
Arrays.sort(sa, Collections.reverseOrder());
System.out.println("Reverse sort: " + Arrays.toString(sa));
Arrays.sort(sa,String.CASE_INSENSITIVE_ORDER);//忽略大小写
System.out.println("Case-insensitive sort: " + Arrays.toString(sa));
}
}
7.5 在已排序数组中查找元素
public class AlphabeticSearch {
public static void main(String[] args) {
String[] sa = Generated.array(new String[30],new RandomGenerator.String(5));
Arrays.sort(sa,String.CASE_INSENSITIVE_ORDER);
System.out.println(Arrays.toString(sa));
int index = Arrays.binarySearch(sa,sa[10],String.CASE_INSENSITIVE_ORDER);
System.out.println("Index: " + index + "
" + sa[index]);
}
}
8 总结
优选容器而不是数组
在容器引入泛型和自动包装机制之后 数组的优势几乎荡然无存，
除非发现性能问题，才会使用数组，而这种情况也是微乎其微的