【Java】Java创建String时，什么情况放进String Pool？

对Java创建String是否放入String pool作代码性的试验。

参考的优秀文章

JAVA面试题解惑系列（二）——到底创建了几个String对象？

public String(String original) API

public String intern() API

版本说明

JDK 1.7.0_71

new String() 与 intern()

package com.nicchagil.commonstudy.No01String的池;

public class Call {
    
    public static void main(String[] args) {
        String a = "abc";
        String b = "abc";
        String c = new String(a);
        String d = "ab" + "c";
        String e = "abc".intern();
        String f = new String(a).intern();
        
        System.out.println("a == b : " + (a == b));
        System.out.println("a == c : " + (a == c));
        System.out.println("a == d : " + (a == d));
        System.out.println("a == e : " + (a == e));
        System.out.println("a == f : " + (a == f));
    }

}

日志

a == b : true
a == c : false
a == d : true
a == e : true
a == f : true

变量、常量字符串拼接，他们是否放入String Pool？

package com.nicchagil.commonstudy.No01String的池;

public class Call2 {

    public static void main(String[] args) {
        final String A = "123";
        final String B = "456";
        String C = "123456";
        String a = "123";
        String b = "456";
        
        System.out.println("C == (A + B) : " + (C == (A + B))); // A、B为final类型
        System.out.println("C == ("123" + "456") : " + (C == ("123" + "456"))); // "123"、"456"为常量
        System.out.println("C == (a + b) : " + (C == (a + b))); // a、b为变量
        System.out.println("C == (a + "456") : " + (C == (a + "456"))); // a为变量
        System.out.println("C == (a + b).intern() : " + (C == (a + b).intern())); // 使用intern()方法
    }

}

日志

C == (A + B) : true
C == ("123" + "456") : true
C == (a + b) : false
C == (a + "456") : false
C == (a + b).intern() : true

注意

本日志打印的结果为上文JDK所运行的结果。