测试1:
先看一组String类型比较,废话不多说,直接上代码:
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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(); 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