測(cè)試1：
先看一組String類型比較，廢話不多說，直接上代碼：
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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(); 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：
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a == ba.equals(b)a != ca.equals(c)a == da.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(cè)試2：
這是一組int類型和Integer類型的測(cè)試：
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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"); } }} |
輸出的結(jié)果：
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a == a1b == b1c == c1d != 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。
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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); } |
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。
原文鏈接：http://blog.csdn.net/qq_34988304/article/details/78711473
