Integer.parseInt(String s) 和 Integer.valueOf(String s) 的区别

parseInt源码：

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

    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;
    }

当使用 public static int parseInt(String s) 方法的时候会去调用public static int parseInt(String s, int radix)，这个方法可以将字符串解析成不同进制的int类型的数， 直接调用public static int parseInt(String s)会默认10进制。

---

valueOf源码：

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

如果使用public static Integer valueOf(String s) 方法，两者的区别就在于返回值类型不同，parseInt返回值是int类型，valueOf返回值类型是Integer类型。

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

    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() {}
    }

调用public static Integer valueOf(int i) 方法是，首先会调用IntegerCache()， IntegerCache()其源码中为 [-128 , 128) 之间的Integer对象进行了缓存以提高效率。

下面这题可以帮助理解：

    Integer a = 130;
    Integer b = Integer.valueOf(130);
    System.out.println(a == b); // 结果为false;

    Integer c = 20;
    Integer d = Integer.valueOf(20);
    System.out.println(c == d); // 结果为true;

原因是：如果整数的范围在[-128 , 128) 之间，会直接从缓存中取值，而如果整数的范围不在[-128 , 128)之间会在堆内存中new出一个新的Integer类型的对象。