public final class String //String类不可继承，实现了序列化
implements java.io.Serializable, Comparable<String>, CharSequence {
private final char value[]; //String底层就是一个char数组
private int hash; // Default to 0
private static final long serialVersionUID = -6849794470754667710L;
private static final ObjectStreamField[] serialPersistentFields =
new ObjectStreamField[0];
public String() { //默认返回空字符串
this.value = "".value;
}
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
public int compareTo(String anotherString) {
//当前字符串v1和另一串v2比较，根据两者最小长度遍历，逐个字符进行比较，若发现不同的字符串就返回v1[k]-v2[k],
//若遍历完发现全部相同，则返回v1.len-v2.len
int len1 = value.length;
int len2 = anotherString.value.length;
int lim = Math.min(len1, len2);
char v1[] = value;
char v2[] = anotherString.value;
int k = 0;
while (k < lim) {
char c1 = v1[k];
char c2 = v2[k];
if (c1 != c2) {
return c1 - c2;
}
k++;
}
return len1 - len2;
}
public boolean equals(Object anObject) {
//首先比较两者的内存地址，若相同则返回true；
//若anObject属于String类型，则转换为String类型，在两者的长度相等的前提下，逐个字符进行比较。
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = value.length;
if (n == anotherString.value.length) {
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
while (n-- != 0) {
if (v1[i] != v2[i])
return false;
i++;
}
return true;
}
}
return false;
}
public static String valueOf(Object obj) {
/*
String a1 = "无论字符串是多少，使用==比较时，永远返回true";
String a2 = "无论字符串是多少，使用==比较时，永远返回true";
String a3 = null;
System.out.println(a1==a2); // true
System.out.println("a1:"+a1.hashCode()+" a2:"+a2.hashCode()); //a1:-1368761767 a2:-1368761767
System.out.println("若给a3赋值null，则返回一个字符串'null' :"+a3);//若给a3赋值null，则返回一个字符串'null' :null
String b1 = new String("aa");//若使用new String()声明，使用==返回false，因为地址不同
String b2 = new String("aa");
System.out.println(b1 == b2);// false
System.out.println("b1:"+b1.hashCode()+" b2:"+b2.hashCode()); //b1:3104 b2:3104
*/
return (obj == null) ? "null" : obj.toString();
}
public static String valueOf(char data[]) {
return new String(data);
}
public int hashCode() {
/*
这里计算字符串的hash码如下，hash默认为0，若字符串长度大于0，则遍历字符串，算法公式：h = 31 * h + val[i]
*/
int h = hash;
if (h == 0 && value.length > 0) {
char val[] = value;
for (int i = 0; i < value.length; i++) {
h = 31 * h + val[i];
}
hash = h;
}
return h;
}
}
public final class StringBuffer //StringBuffer和StringBulider最大的区别是前者线程安全，后者线程不安全，其他的功能基本相同，
//两者都继承了AbstractStringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence{
@Override
public synchronized StringBuffer append(String str) {
toStringCache = null;
super.append(str);
return this;
}
@Override
public synchronized String toString() {
if (toStringCache == null) {
toStringCache = Arrays.copyOfRange(value, 0, count);
}
return new String(toStringCache, true);
}
}
public final class StringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence{
@Override
public synchronized StringBuffer append(String str) {
toStringCache = null;
super.append(str);
return this;
}
@Override
public String toString() {
// Create a copy, don't share the array
return new String(value, 0, count);
}
}
abstract class AbstractStringBuilder implements Appendable, CharSequence {
char[] value;
int count;
AbstractStringBuilder() {
}
AbstractStringBuilder(int capacity) {
value = new char[capacity];
}
public AbstractStringBuilder append(String str) {
if (str == null)
return appendNull();
int len = str.length();
ensureCapacityInternal(count + len);
str.getChars(0, len, value, count);
count += len;
return this;
/*String类的getChars()
public void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) {
if (srcBegin < 0) {
throw new StringIndexOutOfBoundsException(srcBegin);
}
if (srcEnd > value.length) {
throw new StringIndexOutOfBoundsException(srcEnd);
}
if (srcBegin > srcEnd) {
throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);
}
System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
}
*/
}
private void ensureCapacityInternal(int minimumCapacity) {
// overflow-conscious code
if (minimumCapacity - value.length > 0)
expandCapacity(minimumCapacity);
}
void expandCapacity(int minimumCapacity) {//数组扩容
int newCapacity = value.length * 2 + 2;
if (newCapacity - minimumCapacity < 0)
newCapacity = minimumCapacity;
if (newCapacity < 0) {
if (minimumCapacity < 0) // overflow
throw new OutOfMemoryError();
newCapacity = Integer.MAX_VALUE;
}
value = Arrays.copyOf(value, newCapacity);
}
}
public final class StringBuffer
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence{ //线程安全的
@Override
public synchronized StringBuffer append(String str) {
toStringCache = null;
super.append(str);
return this;
}
@Override
public synchronized String toString() {
if (toStringCache == null) {
toStringCache = Arrays.copyOfRange(value, 0, count);
}
return new String(toStringCache, true);
}
}
public final class StringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence{ //线程不安全，是两者最大的区别，两者的实现代码基本类似
@Override
public synchronized StringBuffer append(String str) {
toStringCache = null;
super.append(str);
return this;
}
@Override
public String toString() {
// Create a copy, don't share the array
return new String(value, 0, count);
}
}
abstract class AbstractStringBuilder implements Appendable, CharSequence {
char[] value;
int count;
AbstractStringBuilder() {
}
AbstractStringBuilder(int capacity) {
value = new char[capacity];
}
public AbstractStringBuilder append(String str) {
if (str == null)
return appendNull();
int len = str.length();
ensureCapacityInternal(count + len);
str.getChars(0, len, value, count);
count += len;
return this;
}
private void ensureCapacityInternal(int minimumCapacity) {
// overflow-conscious code
if (minimumCapacity - value.length > 0)
expandCapacity(minimumCapacity);
}
void expandCapacity(int minimumCapacity) {
int newCapacity = value.length * 2 + 2;
if (newCapacity - minimumCapacity < 0)
newCapacity = minimumCapacity;
if (newCapacity < 0) {
if (minimumCapacity < 0) // overflow
throw new OutOfMemoryError();
newCapacity = Integer.MAX_VALUE;
}
value = Arrays.copyOf(value, newCapacity);
}
}