Java 面试题分析

 

分析代码如下：

demo1:

 public static void main(String[] args)
    {
        String str1 = "abc";//常量池
        String str2 = "abc";//常量池
        String str3 = new String("abc");//堆 中
        System.out.println(str1 == str2);//同一个直 true
        System.out.println(str1 == str3);//不同 false
        System.out.println(str1.equals(str3));//Hashcode 相同
        System.out.println(str2.equals(str3));//Hashcode 相同
        
        System.out.println();
        Integer i =127,j=127,m=129,n=129;
        System.out.println(i==j);//
        System.out.println(m==n);
    }

　　结果：

true
false
true
true

true
false

Integer 原码如下：

assert IntegerCache.high >= 127;

    /**
     * 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.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

　　

demo2；

public class NULL
{
    public static void test()
    {
        System.out.println("test");
    }

    public static void main(String[] args)
    {
        ((NULL) null).test();//同 NULL.test(); 
        
        NULL n = new NULL();
        n.test(); //NULL.test();
        
        Object obj = 0.6324;
        System.out.println(obj.getClass());//Double

        final StringBuffer a = new StringBuffer("aaa");
        a.append("a");//special
        System.out.println(a);
    }
}