JAVA拆箱与装箱及cache范围
1. 调用Integer.valueOf()时, 对[-128,127]进行了缓存!
public final class Integer extends Number implements Comparable<Integer> {
private static class IntegerCache {
private IntegerCache(){}
static final Integer cache[] = new Integer[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Integer(i - 128);
}
}
public static Integer valueOf(int i) {
final int offset = 128;
if (i >= -128 && i <= 127) { // must cache
return IntegerCache.cache[i + offset];
}
return new Integer(i);
}
}
2. 装箱与拆箱的本质:
装箱: 自动Integer.valueOf();
Integer jjj = 1;
编译后源码:
Integer jjj = Integer.valueOf(1);
拆箱: 自动对象.intValue();
int jjj = new Integer(2);
编译后源码:
int jjj = (new Integer(2)).intValue();
EG:
示例代码:
public class Test {
public static void main(final String[] args) {
Integer a = 100;
Integer b = 100;
System.out.println(a == b);
main2();
main3();
}
public static void main2() {
Integer a = 156;
Integer b = 156;
System.out.println(a == b);
}
public static void main3() {
int a = new Integer(156);
int b = Integer.valueOf(156);
System.out.println(a == b);
}
}
经过装箱拆箱本质代码为:
public class Test
{
public static void main(String[] args)
{
Integer a = Integer.valueOf(100);
Integer b = Integer.valueOf(100);
System.out.println(a == b);
main2();
main3();
}
public static void main2() {
Integer a = Integer.valueOf(156);
Integer b = Integer.valueOf(156);
System.out.println(a == b);
}
public static void main3() {
int a = new Integer(156).intValue();
int b = Integer.valueOf(156).intValue();
System.out.println(a == b);
}
}
运行结果:
true false true
3. yekui说, 查了一下源码,确实如此!
java使用该机制是为了达到最小化数据输入和输出的目的,这是一种优化措施,提高效率.
valueOf缓存范围
其他的包装器:
Boolean： (全部缓存)
Byte： (全部缓存)
Character [0, 127] 缓存
Short [-128, 127] 缓存
Long [-128, 127] 缓存
Float (没有缓存)
Doulbe (没有缓存)