Integer.parseInt(String s) 和 Integer.valueOf(String s) 的差別

parseInt源碼：

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

    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 = ;
        boolean negative = false;
        int i = , len = s.length();
        int limit = -Integer.MAX_VALUE;
        int multmin;
        int digit;

        if (len > ) {
            char firstChar = s.charAt();
            if (firstChar < '0') { // Possible leading "+" or "-"
                if (firstChar == '-') {
                    negative = true;
                    limit = Integer.MIN_VALUE;
                } else if (firstChar != '+')
                    throw NumberFormatException.forInputString(s);

                if (len == ) // 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 < ) {
                    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;
    }
           

當使用 public static int parseInt(String s) 方法的時候會去調用public static int parseInt(String s, int radix)，這個方法可以将字元串解析成不同進制的int類型的數， 直接調用public static int parseInt(String s)會預設10進制。

valueOf源碼：

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

如果使用public static Integer valueOf(String s) 方法，兩者的差別就在于傳回值類型不同，parseInt傳回值是int類型，valueOf傳回值類型是Integer類型。

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 = -;
        static final int high;
        static final Integer cache[];

        static {
            // high value may be configured by property
            int h = ;
            String integerCacheHighPropValue =
                sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
            if (integerCacheHighPropValue != null) {
                try {
                    int i = parseInt(integerCacheHighPropValue);
                    i = Math.max(i, );
                    // Maximum array size is Integer.MAX_VALUE
                    h = Math.min(i, Integer.MAX_VALUE - (-low) -);
                } catch( NumberFormatException nfe) {
                    // If the property cannot be parsed into an int, ignore it.
                }
            }
            high = h;

            cache = new Integer[(high - low) + ];
            int j = low;
            for(int k = ; k < cache.length; k++)
                cache[k] = new Integer(j++);

            // range [-128, 127] must be interned (JLS7 5.1.7)
            assert IntegerCache.high >= ;
        }

        private IntegerCache() {}
    }
           

調用public static Integer valueOf(int i) 方法是，首先會調用IntegerCache()， IntegerCache()其源碼中為 [-128 , 128) 之間的Integer對象進行了緩存以提高效率。

下面這題可以幫助了解：

Integer a = ;
    Integer b = Integer.valueOf();
    System.out.println(a == b); // 結果為false;

    Integer c = ;
    Integer d = Integer.valueOf();
    System.out.println(c == d); // 結果為true;
           

原因是：如果整數的範圍在[-128 , 128) 之間，會直接從緩存中取值，而如果整數的範圍不在[-128 , 128)之間會在堆記憶體中new出一個新的Integer類型的對象。