Java基本数据类型四类八种,四类:整型、浮点型、字符型、布尔型;八种:byte、short、int、long、double、float、char、boolean。
对应的包装类:Byte、Short、Integer、Long、Double、Float、Character、Boolean。
jdk1.5之后,基本数据类型和包装类之间就实现了自动拆装箱的功能。
自动装箱,原理是包装类的valueOf(xxx)方法,自动拆箱则是调用包装类的xxxValue()方法。
示例:
public class WrapperTest {
public static void main(String[] args){
test(100);
test(128);
boxingTest(100);
}
private static void boxingTest(int i){
Integer num = i;
int n = num;
}
private static void test(int i){
Integer num1 = i;
Integer num2 = i;
System.out.println(String.format("Integer %d==%d result: " + (num1 == num2), i, i));
System.out.println(String.format("Integer %d.equals(%d) result: " + num1.equals(num2), i, i));
}
}
/* 执行结果
Integer 100==100 result: true
Integer 100.equals(100) result: true
Integer 128==128 result: false
Integer 128.equals(128) result: true
*/
证明方法,编译反编译后可以得到:
public class WrapperTest
{
public static void main(String[] paramArrayOfString)
{
test(100);
test(128);
boxingTest(100);
}
private static void boxingTest(int paramInt) {
//自动装箱
Integer localInteger = Integer.valueOf(paramInt);
//自动拆箱
int i = localInteger.intValue();
}
private static void test(int paramInt) {
Integer localInteger1 = Integer.valueOf(paramInt);
Integer localInteger2 = Integer.valueOf(paramInt);
System.out.println(String.format(new StringBuilder().append("Integer %d==%d result: ").append(localInteger1 == localInteger2).toString(), new Object[] { Integer.valueOf(paramInt), Integer.valueOf(paramInt) }));
System.out.println(String.format(new StringBuilder().append("Integer %d.equals(%d) result: ").append(localInteger1.equals(localInteger2)).toString(), new Object[] { Integer.valueOf(paramInt), Integer.valueOf(paramInt) }));
}
}
以上示例同时引出一个基本原理:包装类的缓存值,以及包装类重写了equals方法。
下面是Integer的源代码:
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
public boolean equals(Object obj) {
if (obj instanceof Integer) {
return value == ((Integer)obj).intValue();
}
return false;
}
Integer缓存值范围:-128 ~ 127,源代码中也有说明:
/**
* 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() {}
}
总结各包装类缓存值范围:
Byte: -128 ~ 127
Character: 0 ~ 127
Short:-128 ~ 127
Integer:-128 ~ 127
Long:-128 ~ 127
Boolean:true、false
Double和Float没有缓存值,都是直接new的对象。