Integer使用雙等號比較會發生什麼
阿新 • • 發佈:2018-12-11
話不多說,根據以下程式執行,列印的結果為什麼不同?
Integer a = 100;
Integer b = 100;
System.out.println(a == b);//print : true
Integer a = 200;
Integer b = 200;
System.out.println(a == b);//print : false
以上兩段程式碼在執行後為什麼會出現這樣神奇的效果,糾察原因在Integer的原始碼中一段特殊的程式碼,在程式碼中會在虛擬機器載入時將-128至127的資料提前快取到記憶體中,所以會導致雙等號比較一致的問題。
/** * 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 {@code -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) { try { 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); } catch( NumberFormatException nfe) { // If the property cannot be parsed into an int, ignore it. } } high = h; cache = new Integer[(high - low) + 1]; int j = low; for(int k = 0; k < cache.length; k++) cache[k] = new Integer(j++); // range [-128, 127] must be interned (JLS7 5.1.7) assert IntegerCache.high >= 127; } private IntegerCache() {} }