1.静态内部类IntegerCache

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() {} }

缓存池的存在保证了【-128，127】之间的数字会使用缓存池，在这种情况下

public static void main(String[] args) { Integer a = 128; Integer b = 128; System.out.println(a == b); System.out.println(a.equals(b)); Integer a1 = 1; Integer b1 = 1; System.out.println(a1 == b1); } 输出结果： false true true

 

Integer类重写了equals方法

public boolean equals(Object obj) { if (obj instanceof Integer) { return value == ((Integer)obj).intValue(); } return false; } public int intValue() { return value; }

Integer类的hashcode方法 返回原值

@Override public int hashCode() { return Integer.hashCode(value); } public static int hashCode(int value) { return value; }

比较高级的是Integer类的decode方法以及转成数字的一些方法，以decode方法为例

 

public static Integer decode(String nm) throws NumberFormatException { int radix = 10; int index = 0; boolean negative = false; Integer result; if (nm.length() == 0) throw new NumberFormatException("Zero length string"); char firstChar = nm.charAt(0); // Handle sign, if present if (firstChar == '-') { negative = true; index++; } else if (firstChar == '+') index++; // Handle radix specifier, if present if (nm.startsWith("0x", index) || nm.startsWith("0X", index)) { index += 2; radix = 16; } else if (nm.startsWith("#", index)) { index ++; radix = 16; } else if (nm.startsWith("0", index) && nm.length() > 1 + index) { index ++; radix = 8; } if (nm.startsWith("-", index) || nm.startsWith("+", index)) throw new NumberFormatException("Sign character in wrong position"); try { result = Integer.valueOf(nm.substring(index), radix); result = negative ? Integer.valueOf(-result.intValue()) : result; } catch (NumberFormatException e) { // If number is Integer.MIN_VALUE, we'll end up here. The next line // handles this case, and causes any genuine format error to be // rethrown. String constant = negative ? ("-" + nm.substring(index)) : nm.substring(index); result = Integer.valueOf(constant, radix); } return result; }

里面调用了Integer的valueOf方法，如果没有radis禁止参数，则默认10进制

public static Integer valueOf(String s, int radix) throws NumberFormatException { return Integer.valueOf(parseInt(s,radix)); } public static Integer valueOf(String s) throws NumberFormatException { return Integer.valueOf(parseInt(s, 10)); }

里面调用parseInt方法

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; }