JDK7的新特性

本篇博客的内容

　　一、二进制字面量
　　二、数字字面量可以出现下划线
　　三、switch 语句可以用字符串
　　四、泛型简化
　　五、异常的多个catch合并
　　六、try-with-resources 语句

一、二进制字面量  <=返回目录

// 二进制字面量
int x = 0b100101;
System.out.println(x); // 37

二、数字字面量可以出现下划线  <=返回目录

// 数字字面量可以出现下划线
int x = 1_000;
int y = 1_1123_1000;
int z = 1_2_3_4_5;
System.out.println("x=" + x + ", y=" + y + ",z=" + z);

// 不能出现在进制标识和数值之间
int a = 0b0000_0011;
// 不能出现在数值开头和结尾
int b = 0x11_22;
// 不能出现在小数点旁边
double c = 12.3_4;
System.out.println("a=" + a + ", b=" + b + ",c=" + c);

三、switch 语句可以用字符串  <=返回目录

package com.oy.test;

public class Test {

    public static void main(String[] args) {

        String key = "KEY_1";
        String value = getValue(key);
        System.out.println("key=" + key + ", value=" + value);
    }

    private static String getValue(String key) {
        String value = "";
        
        switch (key) {
        case "KEY_1":
            value += "格式错误";
            break;
        case "KEY_2":
            value += "密码错误";
            break;
        default:
            value += "系统错误";
            break;
        }
        
        return value;
    }
}

四、泛型简化  <=返回目录

// 菱形语法：泛型推断
List<String> list = new ArrayList<>();

五、异常的多个catch合并  <=返回目录

// jdk7之前的做法
private static void method() {
    String className = "com.oy.test.Person";
    Class<?> clazz = null;
    try {
        clazz = Class.forName(className);
        Person p = (Person) clazz.newInstance();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (InstantiationException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    }
}

// jdk7之后，异常的多个catch合并
private static void method2() {
    String className = "com.oy.test.Person";
    Class<?> clazz = null;
        try {
            clazz = Class.forName(className);
            Person p = (Person) clazz.newInstance();
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException e) {
            e.printStackTrace();
        }
}

六、try-with-resources 语句  <=返回目录

public static void main(String[] args) {
    method();
}

// jdk7之前的做法
private static void method() {
    // try-with-resources 语句
    // try(必须是java.lang.AutoCloseable的子类对象){…}

    FileReader fr = null;
    FileWriter fw = null;
    try {
        fr = new FileReader("d:/a.txt");
        fw = new FileWriter("d:/b.txt");
        int ch = 0;
        while ((ch = fr.read()) != -1) {
            fw.write(ch);
        }
        // int a = 1/0; // for test
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        if (fw != null) {
            try {
                fw.close();
                System.out.println("close fw...");
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        if (fr != null) {
            try {
                fr.close();
                System.out.println("close fr ...");
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

// jdk7之后改进版的代码
private static void method2() {
    // try-with-resources 语句
    // try(必须是java.lang.AutoCloseable的子类对象){…}
    try (FileReader fr = new FileReader("d:/a.txt"); FileWriter fw = new FileWriter("d:/b.txt");) {
        int ch = 0;
        while ((ch = fr.read()) != -1) {
            fw.write(ch);
        }
        // int a = 1/0; // for test
    } catch (IOException e) {
        e.printStackTrace();
    }
}