阅读目录

• 建议26：提防包装类型的null值
• 建议27：谨慎包装类型的大小比较
• 建议28：优先使用整型池
• 建议29：优先选择基本类型
• 建议30：不要随便设置随机种子

 1 import java.util.ArrayList;
 2 import java.util.List;
 3 
 4 public class Client26 {
 5 
 6     public static int testMethod(List<Integer> list) {
 7         int count = 0;
 8         for (int i : list) {
 9             count += i;
10         }
11         return count;
12     }
13 
14     public static void main(String[] args) {
15         List<Integer> list = new ArrayList<Integer>();
16         list.add(1);
17         list.add(2);
18         list.add(null);
19         System.out.println(testMethod(list));
20     }
21 }

public static int testMethod(List<Integer> list) {
        int count = 0;
        for (Integer i : list) {
            count += (i != null) ? i : 0;
        }
        return count;
    }

 1 public class Client27 {
 2     public static void main(String[] args) {
 3         Integer i = new Integer(100);
 4         Integer j = new Integer(100);
 5         compare(i, j);
 6     }
 7 
 8     public static void compare(Integer i, Integer j) {
 9         System.out.println(i == j);
10         System.out.println(i > j);
11         System.out.println(i < j);
12 
13     }
14 }

 1 import java.util.Scanner;
 2 
 3 public class Client28 {
 4     public static void main(String[] args) {
 5         Scanner input = new Scanner(System.in);
 6         while (input.hasNextInt()) {
 7             int tempInt = input.nextInt();
 8             System.out.println("\n=====" + tempInt + " 的相等判断=====");
 9             // 两个通过new产生的对象
10             Integer i = new Integer(tempInt);
11             Integer j = new Integer(tempInt);
12             System.out.println(" new 产生的对象：" + (i == j));
13             // 基本类型转换为包装类型后比较
14             i = tempInt;
15             j = tempInt;
16             System.out.println(" 基本类型转换的对象：" + (i == j));
17             // 通过静态方法生成一个实例
18             i = Integer.valueOf(tempInt);
19             j = Integer.valueOf(tempInt);
20             System.out.println(" valueOf产生的对象：" + (i == j));
21         }
22     }
23 }

 1  /**
 2      * Returns an {@code Integer} instance representing the specified
 3      * {@code int} value.  If a new {@code Integer} instance is not
 4      * required, this method should generally be used in preference to
 5      * the constructor {@link #Integer(int)}, as this method is likely
 6      * to yield significantly better space and time performance by
 7      * caching frequently requested values.
 8      *
 9      * This method will always cache values in the range -128 to 127,
10      * inclusive, and may cache other values outside of this range.
11      *
12      * @param  i an {@code int} value.
13      * @return an {@code Integer} instance representing {@code i}.
14      * @since  1.5
15      */
16     public static Integer valueOf(int i) {
17         assert IntegerCache.high >= 127;
18         if (i >= IntegerCache.low && i <= IntegerCache.high)
19             return IntegerCache.cache[i + (-IntegerCache.low)];
20         return new Integer(i);
21     }

 1  /**
 2      * Cache to support the object identity semantics of autoboxing for values between
 3      * -128 and 127 (inclusive) as required by JLS.
 4      *
 5      * The cache is initialized on first usage.  The size of the cache
 6      * may be controlled by the -XX:AutoBoxCacheMax=<size> option.
 7      * During VM initialization, java.lang.Integer.IntegerCache.high property
 8      * may be set and saved in the private system properties in the
 9      * sun.misc.VM class.
10      */
11 
12     private static class IntegerCache {
13         static final int low = -128;
14         static final int high;
15         static final Integer cache[];
16 
17         static {
18             // high value may be configured by property
19             int h = 127;
20             String integerCacheHighPropValue =
http://www.mamicode.com/21                 sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
22             if (integerCacheHighPropValue != null) {
23                 int i = parseInt(integerCacheHighPropValue);
24                 i = Math.max(i, 127);
25                 // Maximum array size is Integer.MAX_VALUE
26                 h = Math.min(i, Integer.MAX_VALUE - (-low));
27             }
28             high = h;
29 
30             cache = new Integer[(high - low) + 1];
31             int j = low;
32             for(int k = 0; k < cache.length; k++)
33                 cache[k] = new Integer(j++);
34         }
35 
36         private IntegerCache() {}
37     }

 1 public class Client29 {
 2     public static void main(String[] args) {
 3         Client29 c = new Client29();
 4         int i = 140;
 5         // 分别传递int类型和Integer类型
 6         c.testMethod(i);
 7         c.testMethod(new Integer(i));
 8     }
 9 
10     public void testMethod(long a) {
11         System.out.println(" 基本类型的方法被调用");
12     }
13 
14     public void testMethod(Long a) {
15         System.out.println(" 包装类型的方法被调用");
16     }
17 }

 1 public class Client29 {
 2     public static void main(String[] args) {
 3         Client29 c = new Client29();
 4         int i = 140;
 5         c.testMethod(i);
 6     }
 7 
 8     public void testMethod(Long a) {
 9         System.out.println(" 包装类型的方法被调用");
10     }
11 }

 1 import java.util.Random;
 2 
 3 public class Client30 {
 4     public static void main(String[] args) {
 5         Random r = new Random();
 6         for(int i=1; i<=4; i++){
 7             System.out.println("第"+i+"次:"+r.nextInt());
 8             
 9         }
10     }
11 }

1 public class Client30 {
2     public static void main(String[] args) {
3         Random r = new Random(1000);
4         for(int i=1; i<=4; i++){
5             System.out.println("第"+i+"次:"+r.nextInt());
6             
7         }
8     }
9 }