Integer.valueOf()

最新推荐文章于 2024-05-21 16:24:16 发布

转载最新推荐文章于 2024-05-21 16:24:16 发布 · 1.1k 阅读

本文探讨了Java中自动装箱机制的工作原理，特别是Integer类的缓存使用方式及其对对象比较的影响。通过具体示例展示了不同数值范围内的Integer对象比较结果，并深入分析了JDK源码。

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public class AutoBoxDemo {

    public static void main(String[] args) {
        System.out.println(Integer.valueOf("127") == Integer.valueOf("127"));
        System.out.println(Integer.valueOf("128") == Integer.valueOf("128"));
        System.out.println(Integer.parseInt("128") == Integer.valueOf("128"));
        System.out.println(Integer.valueOf(127) == Integer.valueOf(127));
        System.out.println(Integer.valueOf(128) == Integer.valueOf(128));
        Integer a = 128, b = 128;
        System.out.println(a == b);


    }

}

true
false
true
true
false
false

以下贴JDK源码//jdk1.8

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

 public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }
    //如果在-128~127是没问题的，当出了这个就返回一个new Integer(i)
    /* static final int low = -128;
        static final int high;*/  
         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;
    }

Integer.parseInt("128")==Integer.valueOf("128");
上面的表达式比较的是左边的原始int值128与右边新创建的Integer对象。但是因为int和Integer之间比较是没有意义的，所以Java在进行比较前会将Integer自动拆箱，所以最后进行的是int和int值之间的比较。由于128和自己相等，所以返回true。