自動裝箱與自動拆箱的一些問題

今天打算複習一下Java基礎，之前學的太快速了，現在暑假，好好把那些細節看一下

複習到自動裝箱和自動拆箱的時候，這裏有個很有趣的現象

Integer n1 = 100;
Integer n2 = 100;
System.out.println(n1 == n2);

Integer n3 = 200;
Integer n4 = 200;
System.out.println(n3 == n4);

你們猜猜結果是什麼  第一個是true，第二個是false。有趣吧。

其實它們是相當於

Integer n1 = Integer.valueOf(100);
Integer n2 = Integer.valueOf(100);
System.out.println(n1 == n2);

Integer n1 = Integer.valueOf(200);
Integer n2 = Integer.valueOf(200);
System.out.println(n1 == n2);

所以那個valueOf()就是問題的關鍵啦，看一下這個函數的源碼

/**
  * 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) {
     assert IntegerCache.high >= 127;
  if (i >= IntegerCache.lo&&i<=IntegerCache.high)
 return IntegerCache.cache[i + (-IntegerCache.low)];
     return new Integer(i);
 }

所以我們知道了，對於-128到127之間的數,Integer.valueOf(99)返回的是緩存中的對象,所以兩次調用valueOf返回的是同一個對象,故結果是true.而Integer.valueOf(199)返回的則是重新實例化的對象,故結果是false.

 

自動拆箱

 

Integer n1 = 10;
int t1 = n1;
Integer n2 = 10;
int t2 = n2;
System.out.println(t1 == t2);

 

Integer n1 = 199;
int t1 = n1;
Integer n2 = 199;
int t2 = n2;
System.out.println(t1 == t2);

兩次的結果是true。

 

所以我們要注意自動裝箱時候數值的範圍的選擇~~