String 代码

package blog.string;

import org.junit.Test;

/**
 * intern 定义：
 * 如果常量池中存在当前字符串, 就会直接返回当前字符串. 如果常量池中没有此字符串, 会将此字符串放入常量池中后, 再返回
 * <p>
 * java6 常量池在永久代
 * java7+java8  常量池在堆中 常量池中可以存储堆中的引用
 * Created by zhangshaolin on 2017/12/20.
 */
public class StringIntern {

    @Test
    public void test() {
        String s = new String("1");
        String s2 = "1";
        System.out.println(s == s2); //false

        String s3 = new String("1") + new String("1");
        String s4 = "11";
        System.out.println(s3 == s4);//false
    }

    @Test
    public void test1() {
        String s = new String("1");
        String s2 = "1";
        System.out.println(s == s2);//false

        String s3 = new String("1") + new String("1");
        s3.intern();
        String s4 = "11";
        System.out.println(s3 == s4);//false
    }

    @Test
    public void test2() {
        String s = new String("1");
        String s2 = "1";
        s.intern();
        System.out.println(s == s2);//false

        String s3 = new String("1") + new String("1");
        String s4 = "11";
        s3.intern();
        System.out.println(s3 == s4);//false
    }


    @Test
    public void test3() {
        String s = new String("1");
        String s2 = "1";
        s.intern();
        System.out.println(s == s2);//false

        String s3 = "1" + "1";
        String s4 = "11";
        s3.intern();
        System.out.println(s3 == s4);//true
    }

    @Test
    public void test4() {
        String str1 = "a";
        String str2 = "b";
        String str3 = "ab";
        String str4 = str1 + str2;
        String str5 = new String("ab");
        System.out.println(str5.equals(str3));//true
        System.out.println(str5 == str3);//false
        System.out.println(str5.intern() == str3);//true
        System.out.println(str5.intern() == str4);//false
    }
}

package blog.string;

import org.junit.Test;

/**
 * 方法名、java等字符串在代码执行前已在常量池中存在
 * 所以s2.intern() 返回的是常量池中的数据
 * <p>
 * <p>
 * s1.intern() 返回的是常量池中的引用，引用的对象是堆中对象，所以=s1
 * <p>
 * <p>
 * Created by zhangshaolin on 2017/12/26.
 */
public class StringInternJava {
    @Test
    public void test() {
        String s11 = "a" + "b"; //存在这个前后值不一样
        System.out.println(s11 == "ab");//true
    }

    @Test
    public void testA() {
        String s1 = new String("a") + new String("b");
        //String s11 = "a" + "b"; //存在这个前后值不一样
        /**
         是否存在 String s11 = "a" + "b";
         存在  不存在
         true    true
         false   true
         false   true
         */
        System.out.println(s1.intern() == "ab");
        System.out.println(s1.intern() == s1);
        System.out.println(s1 == "ab");

        String s2 = new String("ja") + new String("va");
        System.out.println(s2.intern() == "java");//true
        System.out.println(s2.intern() == s2);//false
        System.out.println(s2 == "java");//false
    }

    @Test
    public void testB() {
        String s2 = new String("java");
        String s3 = new StringBuilder().append("ja").append("va").toString();

        System.out.println(s2.intern() == s2);//false
        System.out.println(s2.intern() == s3);//false
    }

    @Test
    public void test1() {
        String s1 = new StringBuilder().append("aa").append("bb").toString();
        System.out.println(s1.intern() == s1); //true
        String s2 = new StringBuilder().append("ja").append("va").toString();
        System.out.println(s2.intern() == s2);//false
    }

    @Test
    public void test3() {
        String s1 = new String("aa") + new String("bb");
        System.out.println(s1.intern() == s1); //true
    }

    @Test
    public void test2() {
        String s1 = new String("aabb");
        System.out.println(s1.intern() == s1);//false
        String s2 = new StringBuilder().append("ja").append("va").toString();
        System.out.println(s2.intern() == s2);//false
    }


}