基本类型
类型 | 字节数 |
---|---|
boolean | ~ |
byte | 1 |
short | 2 |
int | 4 |
long | 8 |
chart | 2 |
float | 4 |
double | 8 |
boolean有两个值:true和false,但是具体大小没有确定
对于boolean类型,会转换为int;对于boolean数组,会用byte数组来实现
包装类型
基本类型 | 包装类型 |
---|---|
boolean | Boolean |
byte | Byte |
short | Short |
int | Integer |
long | Long |
chart | Character |
float | Float |
double | Double |
基本类型与其对应的包装类型之间的赋值使用自动装箱和拆箱完成。
Integer i = 2; // 装箱
int j = i; // 拆箱
等价于
Integer i = Integer.valueOf(2);
int j = i.intValue();
public final class Integer extends Number implements Comparable<Integer> {
private final int value;
// 其他属性和方法
}
包装类型包含一个final类型的基本类型,并且提供了一些其他的方法
缓冲池
new Integer(123)
和Integer.valueOf(123)
的区别在于:
new Integer(123)
每次都会创建一个新对象Integer.valueOf(123)
会使用缓冲池中的对象
以前一直以为这些缓冲对象都是由JVM管理的,看了源码才知道是由Java代码管理的。
public final class Boolean implements java.io.Serializable,
Comparable<Boolean> {
public static final Boolean TRUE = new Boolean(true);
public static final Boolean FALSE = new Boolean(false);
public static Boolean valueOf(boolean b) {
return (b ? TRUE : FALSE);
}
}
public final class Byte extends Number implements Comparable<Byte> {
private static class ByteCache {
private ByteCache(){}
static final Byte cache[] = new Byte[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Byte((byte)(i - 128));
}
}
public static Byte valueOf(byte b) {
final int offset = 128;
return ByteCache.cache[(int)b + offset];
}
}
public final class Short extends Number implements Comparable<Short> {
private static class ShortCache {
private ShortCache(){}
static final Short cache[] = new Short[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Short((short)(i - 128));
}
}
public static Short valueOf(short s) {
final int offset = 128;
int sAsInt = s;
if (sAsInt >= -128 && sAsInt <= 127) { // must cache
return ShortCache.cache[sAsInt + offset];
}
return new Short(s);
}
}
public final class Integer extends Number implements Comparable<Integer> {
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() {}
}
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
}
public final class Long extends Number implements Comparable<Long> {
private static class LongCache {
private LongCache(){}
static final Long cache[] = new Long[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Long(i - 128);
}
}
public static Long valueOf(long l) {
final int offset = 128;
if (l >= -128 && l <= 127) { // will cache
return LongCache.cache[(int)l + offset];
}
return new Long(l);
}
}
public final class Character implements java.io.Serializable, Comparable<Character> {
private static class CharacterCache {
private CharacterCache(){}
static final Character cache[] = new Character[127 + 1];
static {
for (int i = 0; i < cache.length; i++)
cache[i] = new Character((char)i);
}
}
public static Character valueOf(char c) {
if (c <= 127) { // must cache
return CharacterCache.cache[(int)c];
}
return new Character(c);
}
}
public final class Float extends Number implements Comparable<Float> {
public static Float valueOf(float f) {
return new Float(f);
}
}
public final class Double extends Number implements Comparable<Double> {
public static Double valueOf(double d) {
return new Double(d);
}
}
可以看到对应的缓存池如下:
- Boolean:true 和 false
- Byte:[-128, 127] 也就是全部
- Short:[-128, 127]
- Integer:[-128, 127] (默认)
- Long:[-128, 127]
- Character:[0, 127]
- Float :没有缓存
- Double :没有缓存
参考