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代碼詳解java里的“==”和“equels”區(qū)別

更新時(shí)間：2018年02月10日 15:37:18 作者：Haker_楓

本篇文章通過實(shí)例代碼給大家詳細(xì)解釋了java里的“==”和“equels”區(qū)別，對(duì)此有興趣的朋友跟著小編一起學(xué)習(xí)下。

測試1：

先看一組String類型比較，廢話不多說，直接上代碼：

public class Test {

  public static void main(String[] args) {
    String a = "java書苑";
    String b = "java書苑";
    String c = new String("java書苑");
    String d = new String("java書苑").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)");
    }
  }
}

輸出結(jié)果：

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

總結(jié)：

結(jié)果a == b：程序在運(yùn)行的時(shí)候會(huì)創(chuàng)建一個(gè)字符串緩沖池,在String a = “java書苑”時(shí)， “java書苑”被放到了字符串緩沖池中，當(dāng) String b = “java書苑” 創(chuàng)建字符串的時(shí)候，程序首先會(huì)在這個(gè)String緩沖池中尋找相同值的對(duì)象，所以在b被創(chuàng)建的時(shí)候，程序找到了具有相同值的a,將b 引用 a 所引用的對(duì)象。所以a和b引用的同一個(gè)對(duì)象，故a == b。

結(jié)果a != c:String c = new String(“java書苑”)時(shí)new了一個(gè)新的對(duì)象，故不從String緩沖池尋找，二十直接創(chuàng)建一個(gè)新的對(duì)象。所以a != c。

結(jié)果a == d :當(dāng)調(diào)用 intern 方法時(shí)，如果池已經(jīng)包含一個(gè)等于此 String 對(duì)象的字符串（該對(duì)象由 equals(Object) 方法確定），則返回池中的字符串。否則，將此 String 對(duì)象添加到池中，并且返回此 String 對(duì)象的引用。所有d調(diào)用的同樣是a的對(duì)象。
equals比較的是值，故值一樣時(shí)便相等。

測試2：

這是一組int類型和Integer類型的測試：

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");
    }
  }
}

輸出的結(jié)果：

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

結(jié)果”a == a1”和”b == b1”:int 是基本類型，直接存數(shù)值，而integer是對(duì)象，用一個(gè)引用指向這個(gè)對(duì)象，多以比較的時(shí)候”a == a1”和”b == b1”。

結(jié)果“c == c1”和“d != d1”這里可能有人會(huì)有疑問,為什么“d != d1”.我們一起看一下Integer的源碼。

/**
   * 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);
  }

結(jié)論：這里 Integer 會(huì)初始化一個(gè)[-128,127]的常量池，如果數(shù)值在這個(gè)范圍時(shí)，則引用的是同一個(gè)對(duì)象，如果不在這個(gè)范圍，通過源碼可以看出返回的是new了一個(gè)新的對(duì)象： return new Integer(i);

所以，結(jié)果“c == c1”是引用了同一個(gè)對(duì)象,結(jié)果“d != d1”，是new了一個(gè)新的對(duì)象，故不等。