java中Integer與int比較淺談
阿新 • • 發佈：2019-02-17
今天看到一個面試題
測試程式碼如下
public class test {
@Test
public void test(){
Integer a = 300 ;
Integer b = 300;
int c = 300;
System.out.println(a == b);
System.out.println(a == c);
Integer int1 = 100;
Integer int2 = 100;
System.out.println(int1 == int2);
}
}
輸出結果
false
true
true
Process finished with exit code 0
300難道不等於300麼。結果為什麼會這樣呢？
先反編譯看一波
public class test {
public test() {
}
@Test
public void test() {
Integer a = Integer.valueOf(300);
Integer b = Integer.valueOf(300);
int c = 300;
System.out.println(a == b);
System.out.println(a.intValue() == c);
Integer int1 = Integer.valueOf("100");
Integer int2 = Integer.valueOf("100");
System.out.println(int1 == int2);
}
}
可見，通過 Integer a = 300；這種方法新建一個Integer實際上是呼叫 Integer.valueOf()；的方法建立了一個Integer物件，那為什麼a == b結果為false呢，我門繼續看Integer.valueOf();的原始碼
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
這裡冒出來一個IntegerCache類，我們繼續看原始碼
IntegerCache 是 Integer的一個私有靜態內部類，繼續看
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;