hashCode() vs equals() vs ==

== VS equals()

==

• 基础类型: ==比较的是值
• 引用类型: ==比较的是对象的内存地址

equals()

equals()只能比较引用类型,无法比较基础类型.equals()方法在顶级父类Object中,代码如下:

    public boolean equals(Object obj) {
        return (this == obj);
    }

可以看出这个代码就是判断是否是同一对象.那么,当子类重写 equals()往往都是将属性内容相同的对象认为是同一对象,
如果子类不直接或间接重写Object的equals()方法,那么调用的equals()和==相同.

String a = new String("aa"); // a 为一个引用
String b = new String("aa"); // b为另一个引用,对象的内容一样
String aa = "aa"; // 放在常量池中
String bb = "aa"; // 从常量池中查找
System.out.println(aa == bb);// true
System.out.println(a == b);// false
System.out.println(a.equals(b));// true

上面代码的String重写了equals(),代码如下:

public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;
        }
        if (anObject instanceof String) {
            String anotherString = (String)anObject;
            int n = value.length;
            if (n == anotherString.value.length) {
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
                    if (v1[i] != v2[i])
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }

hashCode() VS equals()

• hashCode()函数返回哈希码,确定该对象在哈希表中的索引位置.同样属于Object类中,代码如下:
  public native int hashCode();,native调用C/C++,返回int哈希码.利用哈希码能够快速检索出对象
  如 HashSet/HashMap会先调用hashCode(),如果哈希码不同,直接判断对象不同,否则进行下面的equals()操作,大大减少了equals()的操作,提高执行速度.(C/C++本身比Java执行快,equals()执行也比hashCode()逻辑复杂)
• hashCode()返回值相同也不能认为是同一对象,存在hash冲突
• hashCode 相同,equals()为true才能认为是同一对象

为什么重写 equals() 时必须重写 hashCode() 方法？

• 正面:两对象相等,那么hashCode也相等,equals()也为true
• 反面:重写 equals(),但是不重写hashCode(),会导致equals(),判断是同一个对象,但是哈希码并不同
• 例子: 重写了equals()方法,不重写hashCode(),同一名学生,添加到hashSet中时候,会添加两次,因为你只是通过属性内容判断的是否为同一对象!!!

包装类型的常量池

• Byte/Short/Integer/Long 这 4 种包装类默认创建了数值 [-128，127] 的相应类型的缓存数据
• Character 创建了数值在 [0,127] 范围的缓存数据
• Boolean 直接true/false
  源代码如下:

    private static class ByteCache {
    private ByteCache(){}

    static final Byte cache[] = new Byte[-(-128) + 127 + 1];

    static {
        for(int i = 0; i < cache.length; i++)
            cache[i] = new Byte((byte)(i - 128));
    }
}

    /**
     * Returns a {@code Byte} instance representing the specified
     * {@code byte} value.
     * If a new {@code Byte} instance is not required, this method
     * should generally be used in preference to the constructor
     * {@link #Byte(byte)}, as this method is likely to yield
     * significantly better space and time performance since
     * all byte values are cached.
     *
     * @param  b a byte value.
     * @return a {@code Byte} instance representing {@code b}.
     * @since  1.5
     */
    public static Byte valueOf(byte b) {
        final int offset = 128;
        return ByteCache.cache[(int)b + offset];
    }

    private static class ShortCache {
    private ShortCache(){}

    static final Short cache[] = new Short[-(-128) + 127 + 1];

    static {
        for(int i = 0; i < cache.length; i++)
            cache[i] = new Short((short)(i - 128));
    }
}

    /**
     * Returns a {@code Short} instance representing the specified
     * {@code short} value.
     * If a new {@code Short} instance is not required, this method
     * should generally be used in preference to the constructor
     * {@link #Short(short)}, 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  s a short value.
     * @return a {@code Short} instance representing {@code s}.
     * @since  1.5
     */
    public static Short valueOf(short s) {
        final int offset = 128;
        int sAsInt = s;
        if (sAsInt >= -128 && sAsInt <= 127) { // must cache
            return ShortCache.cache[sAsInt + offset];
        }
        return new Short(s);
    }

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

private static class LongCache {
        private LongCache(){}

        static final Long cache[] = new Long[-(-128) + 127 + 1];

        static {
            for(int i = 0; i < cache.length; i++)
                cache[i] = new Long(i - 128);
        }
    }

    /**
     * Returns a {@code Long} instance representing the specified
     * {@code long} value.
     * If a new {@code Long} instance is not required, this method
     * should generally be used in preference to the constructor
     * {@link #Long(long)}, as this method is likely to yield
     * significantly better space and time performance by caching
     * frequently requested values.
     *
     * Note that unlike the {@linkplain Integer#valueOf(int)
     * corresponding method} in the {@code Integer} class, this method
     * is <em>not</em> required to cache values within a particular
     * range.
     *
     * @param  l a long value.
     * @return a {@code Long} instance representing {@code l}.
     * @since  1.5
     */
    public static Long valueOf(long l) {
        final int offset = 128;
        if (l >= -128 && l <= 127) { // will cache
            return LongCache.cache[(int)l + offset];
        }
        return new Long(l);
    }

上面的缓存代码可以完美解释下面的结果:

Integer a = 10;
Integer b = 10;
System.out.println(a == b);// 输出 true

Float c = 10f;
Float d = 10f;
System.out.println(c == d);// 输出 false

Double e = 1.2;
Double f = 1.2;
System.out.println(e == f);// 输出 false

Integer a = 10;
Integer b = new Integer(10);
System.out.println(a==b);// false
//Integer a = 10;=>Integer a=Integer.valueOf(10);使用常量池中的对象,Integer b = new Integer(10);创建新对象

自动装箱与拆箱

从字节码中，我们发现装箱其实就是调用了 包装类的valueOf()方法，拆箱其实就是调用了 xxxValue()方法。

• Integer i = 1 等价于 Integer i = Integer.valueOf(1)
• int n = i 等价于 int n = i.intValue();
• 如果频繁拆装箱的话，也会严重影响系统的性能。我们应该尽量避免不必要的拆装箱操作。