Java有八种基本类型,分别是 int short long byte float double boolean char。由于基本类型非面向对象,所以在有些时候,比如想使用.toString()就无法使用。因此。就产生了包装类,他们与基本类型相互对应。
int | Integer |
short | Short |
long | Long |
byte | Byte |
float | Float |
double | double |
boolean | Boolean |
char | Character |
现在已经知道了,这些包装类的好处是可以使用一系列的好用的方法。得到了好处,必然也有一些让人苦恼的难以理解的部分。比如自动装箱与自动拆箱。
首先,从简单的代码理解这两个过程
Integer i = 100;//自动装箱 int j = i;//自动拆箱
然后再仔细理解一下。Integer名为包装类,自动将基本的“货物100”放入“包装箱Integer”内,就是自动装箱。相反就是自动拆箱。
对于过程理解了,还有个陷阱需要记忆。但是这里,我觉得不要看其他人的理解,因为这是设计的特点,我们就要从源代码理解,这里如果从中间人的语言去思考的话,很可能进入误区了。
/** * Cache to support the object identity semantics of autoboxing for values between * -128 and 127 (inclusive) as required by JLS. * * The cache is initialized on first usage. The size of the cache * may be controlled by the {@code -XX:AutoBoxCacheMax=<size>} option. * During VM initialization, java.lang.Integer.IntegerCache.high property * may be set and saved in the private system properties in the * sun.misc.VM class. */ 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() {} } /** * Returns an {@code Integer} instance representing the specified * {@code int} value. If a new {@code Integer} instance is not * required, this method should generally be used in preference to * the constructor {@link #Integer(int)}, as this method is likely * to yield significantly better space and time performance by * caching frequently requested values. * * This method will always cache values in the range -128 to 127, * inclusive, and may cache other values outside of this range. * * @param i an {@code int} value. * @return an {@code Integer} instance representing {@code i}. * @since 1.5 */ public static Integer valueOf(int i) { //如果输入的数值大于-128-127,则使用缓存的数组,否则创建新的对象,这对判断两个数值==很有用 if (i >= IntegerCache.low && i <= IntegerCache.high) return IntegerCache.cache[i + (-IntegerCache.low)]; return new Integer(i); }
如果有什么不足,欢迎补充