【面试题】Java String常量相等（==）问题

╰+攻爆jí腚メ 2022-06-14 06:37 160阅读 0赞

# 问题 #

以下三个结果分别输出（true or false）？别小看它，很多程序员因为上面问题出过生产bug

String s3 = "s";
    String s4 = "s";
    System.out.println(s3==s4);
    
    
    ---
    String s5 = "hello";
    String s6 = "he"+"llo";
    System.out.println(s5==s6);
    
    
    ---
    Integer i = 2017;
    Integer j = 2017;
    System.out.println(i==j);
    
    
    ---
    String s1 = new String("s");
    String s2 = new String("s");
    System.out.println(s1==s2);
    System.out.println(s1.intern()==s2.intern());

真正执行结果如下：

String s3 = "s";
    String s4 = "s";
    System.out.println(s3==s4);   //true
    
    
    ---
    String s5 = "hello";
    String s6 = "he"+"llo";
    System.out.println(s5==s6);   //true
    
    
    ---
    Integer i = 2017;
    Integer j = 2017;
    System.out.println(i==j);   //false
    
    
    ---
    String s1 = new String("s");
    String s2 = new String("s");
    System.out.println(s1==s2);   //false
    System.out.println(s1.intern()==s2.intern());   //true

# 解释 #

1.  看看Integer的源代码就知道Integer 把-128-127之间的每个值建立了缓存池，所以Integer i =127，Integer j =127，他们是true，超出就是false。可以参考我的这篇文章：[ 【Java】Integer变量相等（==）比较问题][_Java_Integer]
2.  String s = “s” 是常量池中创建一个对象”s”，所以是true。而String s = new String（”s”）在堆上面分配内存创建一个String对象，栈放了对象引用。如下图：

![20170615170007540][]

但在调用s.intern()方法的时候，会将共享池中的字符串与外部的字符串(s）进行比较,如果共享池存在，返回它，如果不同则将外部字符串放入共享池中，并返回其字符串的引用，这样做的好处就是能够节约空间。

String 的`intern()`方法的官方解释如下：

/**
      * Returns an interned string equal to this string. The VM maintains an internal set of
      * unique strings. All string literals found in loaded classes'
      * constant pools are automatically interned. Manually-interned strings are only weakly
      * referenced, so calling {@code intern} won't lead to unwanted retention.
      *
      * <p>Interning is typically used because it guarantees that for interned strings
      * {@code a} and {@code b}, {@code a.equals(b)} can be simplified to
      * {@code a == b}. (This is not true of non-interned strings.)
      *
      * <p>Many applications find it simpler and more convenient to use an explicit
      * {@link java.util.HashMap} to implement their own pools.
      */
     public native String intern();

s.intern()作用还是很多，比如for循环创建String对象，因为代码事先不并不知道是否存在”hello”或者其他字符串的实例。这样可以节约很多内存空间。

参考资料：

1、[你遇到过哪些质量很高的 Java 面试？][Java]

[_Java_Integer]: http://blog.csdn.net/u010983881/article/details/74943486
[20170615170007540]: /images/20220614/56ebfe23fe3947ddbf1636698a8d7f1e.png
[Java]: https://www.zhihu.com/question/60949531/answer/182458705