代码详解java里的“==”和“equels”区别

时间:2022-09-15 07:29:47

测试1:

先看一组String类型比较,废话不多说,直接上代码:

java" id="highlighter_339495">
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public class Test {
 
  public static void main(String[] args) {
    String a = "java书苑";
    String b = "java书苑";
    String c = new String("java书苑");
    String d = new String("java书苑").intern();
 
    if(a == b){
      System.out.println("a == b");
    }else{
      System.out.println("a != b");
    }
 
    if(a.equals(b)){
      System.out.println("a.equals(b)");
    }else{
      System.out.println("!a.equals(b)");
    }
 
    if(a == c){
      System.out.println("a == c");
    }else{
      System.out.println("a != c");
    }
 
    if(a.equals(c)){
      System.out.println("a.equals(c)");
    }else{
      System.out.println("!a.equals(c)");
    }
 
    if(a == d){
      System.out.println("a == d");
    }else{
      System.out.println("a != d");
    }
 
    if(a.equals(d)){
      System.out.println("a.equals(d)");
    }else{
      System.out.println("a.equals(d)");
    }
  }
}

输出结果:

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a == b
a.equals(b)
a != c
a.equals(c)
a == d
a.equals(d)

总结:

结果a == b:程序在运行的时候会创建一个字符串缓冲池,在String a = “java书苑”时, “java书苑”被放到了字符串缓冲池中,当 String b = “java书苑” 创建字符串的时候,程序首先会在这个String缓冲池中寻找相同值的对象,所以在b被创建的时候,程序找到了具有相同值的a,将b 引用 a 所引用的对象。所以a和b引用的同一个对象,故a == b。

结果a != c:String c = new String(“java书苑”)时new了一个新的对象,故不从String缓冲池寻找,二十直接创建一个新的对象。所以a != c。

结果a == d :当调用 intern 方法时,如果池已经包含一个等于此 String 对象的字符串(该对象由 equals(Object) 方法确定),则返回池中的字符串。否则,将此 String 对象添加到池中,并且返回此 String 对象的引用。所有d调用的同样是a的对象。
equals比较的是值,故值一样时便相等。

测试2:

这是一组int类型和Integer类型的测试:

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public class Test {
 
  public static void main(String[] args) {
 
    int a = 127;
    int a1 = 127;
 
    int b = 128;
    int b1 = 128;
 
 
    Integer c = 127;
    Integer c1 = 127;
 
    Integer d = 128;
    Integer d1 = 128;
 
    if(a == a1){
      System.out.println("a == a1");
    }else{
       System.out.println("a != a1");
    }
 
    if(b == b1){
      System.out.println("b == b1");
    }else{
       System.out.println("b != b1");
    }
 
    if(c == c1){
      System.out.println("c == c1");
    }else{
       System.out.println("c != c1");
    }
 
    if(d == d1){
      System.out.println("d == d1");
    }else{
       System.out.println("d != d1");
    }
  }
}

输出的结果:

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a == a1
b == b1
c == c1
d != d1

结果”a == a1”和”b == b1”:int 是基本类型,直接存数值,而integer是对象,用一个引用指向这个对象,多以比较的时候”a == a1”和”b == b1”。

结果“c == c1”和“d != d1”这里可能有人会有疑问,为什么“d != d1”.我们一起看一下Integer的源码。

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/**
   * Cache to support the object identity semantics of autoboxing for values between
   * -128 and 127 (inclusive) as required by JLS.
   *
   * The cache is initialized on first usage. The size of the cache
   * may be controlled by the -XX:AutoBoxCacheMax=<size> option.
   * During VM initialization, java.lang.Integer.IntegerCache.high property
   * may be set and saved in the private system properties in the
   * sun.misc.VM class.
   */
 
  private static class IntegerCache {
    static final int low = -128;
    static final int high;
    static final Integer cache[];
 
    static {
      // high value may be configured by property
      int h = 127;
      String integerCacheHighPropValue =
        sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
      if (integerCacheHighPropValue != null) {
        int i = parseInt(integerCacheHighPropValue);
        i = Math.max(i, 127);
        // Maximum array size is Integer.MAX_VALUE
        h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
      }
      high = h;
 
      cache = new Integer[(high - low) + 1];
      int j = low;
      for(int k = 0; k < cache.length; k++)
        cache[k] = new Integer(j++);
    }
 
    private IntegerCache() {}
  }
 
  /**
   * Returns an {@code Integer} instance representing the specified
   * {@code int} value. If a new {@code Integer} instance is not
   * required, this method should generally be used in preference to
   * the constructor {@link #Integer(int)}, as this method is likely
   * to yield significantly better space and time performance by
   * caching frequently requested values.
   *
   * This method will always cache values in the range -128 to 127,
   * inclusive, and may cache other values outside of this range.
   *
   * @param i an {@code int} value.
   * @return an {@code Integer} instance representing {@code i}.
   * @since 1.5
   */
  public static Integer valueOf(int i) {
    assert IntegerCache.high >= 127;
    if (i >= IntegerCache.low && i <= IntegerCache.high)
      return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
  }

结论:这里 Integer 会初始化一个[-128,127]的常量池,如果数值在这个范围时,则引用的是同一个对象,如果不在这个范围,通过源码可以看出返回的是new了一个新的对象: return new Integer(i);

所以,结果“c == c1”是引用了同一个对象,结果“d != d1”,是new了一个新的对象,故不等。

原文链接:http://blog.csdn.net/qq_34988304/article/details/78711473