【源码】java包装类总结

1.包装类除了Void和Character，其他六个全部都继承自Number。Number是一个抽象类。如下：

public abstract class Number implements java.io.Serializable {    public abstract int intValue();    public abstract long longValue();    public abstract float floatValue();    public abstract double doubleValue();    public byte byteValue() {        return (byte)intValue();    }    public short shortValue() {        return (short)intValue();    }    private static final long serialVersionUID = -8742448824652078965L;}

2.Integer、Byte、Short、Long内部都有"缓存"静态类,以Integer为例：

缓存即一个Integer数组，默认大小为-128~127。初始化时，会new出256个Integer对象存在此数组中。

 private static class IntegerCache {        static final int low = -128;        static final int high;        static final Integer cache[];        static {            int h = 127;            String integerCacheHighPropValue =                sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");            if (integerCacheHighPropValue != null) {                int i = parseInt(integerCacheHighPropValue);                i = Math.max(i, 127);                // Maximum array size is Integer.MAX_VALUE                h = Math.min(i, Integer.MAX_VALUE - (-low));            }            high = h;            cache = new Integer[(high - low) + 1];            int j = low;            for(int k = 0; k < cache.length; k++)                cache[k] = new Integer(j++);        }        private IntegerCache() {}    }

当调用Integer.valueOf方法时，会首先判断参数是否在缓存范围内，若在，直接返回缓存对象，否则，构造之。

 public static Integer valueOf(int i) {        assert IntegerCache.high >= 127;        if (i >= IntegerCache.low && i <= IntegerCache.high)            return IntegerCache.cache[i + (-IntegerCache.low)];        return new Integer(i);    }

可见，Integer将加载缓存数组延迟到了最晚，避免浪费空间。

另外可以通过intValue方法返回Integer对应的int值（其他包装类类似）:

 public int intValue() {        return value;    }

value是Integer成员变量，构造时被传递进来：

 private final int value;    public Integer(int value) {        this.value = value;    }    public Integer(String s) throws NumberFormatException {        this.value = parseInt(s, 10);    }

3.除了Void类，其余包装类均实现了Comparable接口，可以通过compareTo方法比较两个包装类的大小。同时包装类提供了静态的compare方法比较两个参数的大小（以Integer为例）：

 public int compareTo(Integer anotherInteger) {        return compare(this.value, anotherInteger.value);    }

compareTo方法其实调用了compare静态方法：

public static int compare(int x, int y) {        return (x < y) ? -1 : ((x == y) ? 0 : 1);    }

4.Integer的getInteger方法的作用是返回具有指定名称的系统属性的整数值，不要误用。

public static Integer getInteger(String nm, Integer val) {        String v = null;        try {            v = System.getProperty(nm);        } catch (IllegalArgumentException e) {        } catch (NullPointerException e) {        }        if (v != null) {            try {                return Integer.decode(v);            } catch (NumberFormatException e) {            }        }        return val;    }

5.包装类的toString方法经过了重写

public static String toString(int i) {        if (i == Integer.MIN_VALUE)            return "-2147483648";        int size = (i < 0) ? stringSize(-i) + 1 : stringSize(i);        char[] buf = new char[size];        getChars(i, size, buf);        return new String(buf, true);    }public String toString() {        return toString(value);    }

6.可以使用parseXXX方法将字符串转化为对应的数字。

public static int parseInt(String s, int radix)                throws NumberFormatException    {        /*         * WARNING: This method may be invoked early during VM initialization         * before IntegerCache is initialized. Care must be taken to not use         * the valueOf method.         */        if (s == null) {            throw new NumberFormatException("null");        }        if (radix < Character.MIN_RADIX) {            throw new NumberFormatException("radix " + radix +                                            " less than Character.MIN_RADIX");        }        if (radix > Character.MAX_RADIX) {            throw new NumberFormatException("radix " + radix +                                            " greater than Character.MAX_RADIX");        }        int result = 0;        boolean negative = false;        int i = 0, len = s.length();        int limit = -Integer.MAX_VALUE;        int multmin;        int digit;        if (len > 0) {            char firstChar = s.charAt(0);            if (firstChar < '0') { // Possible leading "+" or "-"                if (firstChar == '-') {                    negative = true;                    limit = Integer.MIN_VALUE;                } else if (firstChar != '+')                    throw NumberFormatException.forInputString(s);                if (len == 1) // Cannot have lone "+" or "-"                    throw NumberFormatException.forInputString(s);                i++;            }            multmin = limit / radix;            while (i < len) {                // Accumulating negatively avoids surprises near MAX_VALUE                digit = Character.digit(s.charAt(i++),radix);                if (digit < 0) {                    throw NumberFormatException.forInputString(s);                }                if (result < multmin) {                    throw NumberFormatException.forInputString(s);                }                result *= radix;                if (result < limit + digit) {                    throw NumberFormatException.forInputString(s);                }                result -= digit;            }        } else {            throw NumberFormatException.forInputString(s);        }        return negative ? result : -result;    } public static int parseInt(String s) throws NumberFormatException {        return parseInt(s,10);    }