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

ゝ一纸荒年。 2022-03-08 15:16 236阅读 0赞

**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。