在Java中==的一個坑。本站提示廣大學習愛好者:(在Java中==的一個坑)文章只能為提供參考,不一定能成為您想要的結果。以下是在Java中==的一個坑正文
察看上面代碼,輸入後果是什麼?
public static void main(String[] args) { Integer p = 10000; Integer q = 10000; System.out.println(p == q); System.out.println(p.equals(q)); }
運轉一次,答案與料想的完全不一樣。在比擬數字的時分,一定要用euqals,不能用==c
檢查Integer的jdk源碼,發現如下片段:
/** * 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() {} }
這兒的IntegerCache有一個靜態的Integer數組,在類加載時就將-128 到 127 的Integer對象創立了,並保管在cache數組中,一旦順序調用valueOf 辦法,假如i的值是在-128 到 127 之間就直接在cache緩存數組中去取Integer對象。
假如是-128 到 127 的值,你不論創立多少個這個范圍內的Integer用ValueOf出來的都是同一個對象。