1.包裝類除了Void和Character，其他六個全部都繼承自Number。Number是一個抽象類。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內部都有"緩存"靜態類,以Integer為例：

緩存即一個Integer數組，默認大小為-128~127。初始化時，會new出256個Integer對象存在此數組中。

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() {}

}

當調用Integer.valueOf方法時，會首先判斷參數是否在緩存范圍內，若在，直接返回緩存對象，否則，構造之。

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);

}

可見，Integer將加載緩存數組延遲到了最晚，避免浪費空間。

另外可以通過intValue方法返回Integer對應的int值(其他包裝類類似):

public int intValue() {

return value;

}

value是Integer成員變量，構造時被傳遞進來：

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類，其余包裝類均實現了Comparable接口，可以通過compareTo方法比較兩個包裝類的大小。同時包裝類提供了靜態的compare方法比較兩個參數的大小(以Integer為例)：public int compareTo(Integer anotherInteger) {

return compare(this.value, anotherInteger.value);

}

compareTo方法其實調用了compare靜態方法：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稱的系統屬性的整數值，不要誤用。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.包裝類的toString方法經過了重寫

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方法將字符串轉化為對應的數字。

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);

}

原文：http://blog.csdn.net/chdjj/article/details/38356325

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