Java源码解析：深入理解==和equals()

1.结论

1.1 ==

1.所有原始数据类型，比较的是值
2.所有的对象比较的是内存地址
注意点：Integer类有对象池[-128,127]

1.2 equals()方法

1.未重写的equals()方法，默认比较内存地址
2.String类和原始数据类型中重写了equals()方法，所有比较的是值，而非内存地址

1.3 建议

因此，要判断两个对象是不是同一个对象，最准确比较方式是==
但若想将属性值相同的两个对象，判断为同一个对象，则建议用重写的equals()方法【比如set集合中去重】

2. 一起看源码

== 是Java内部定义的，没有源码可看，就不管了。这里主要看一下equals（）方法的源码。

原始数据类类型对应的包装类的equals方法中都有一句话如下所示：

The result is {@code true} if and only if the argument is not {@code null} and is a {@code 包装类} object that represents a {@code 对应原始数据类型} with the same value as the {@code 对应原始数据类型} represented by this object.

意思是：只要包装类对应的原始数据类型的值是一样的，那么equals返回true。（前提：对象不为null）

---

Character类

Byte类

Short类

Integer类

Long类

Float类

 /**

     * Compares this object against the specified object.  The result
     * is {@code true} if and only if the argument is not
     * {@code null} and is a {@code Float} object that
     * represents a {@code float} with the same value as the
     * {@code float} represented by this object. For this
     * purpose, two {@code float} values are considered to be the
     * same if and only if the method {@link #floatToIntBits(float)}
     * returns the identical {@code int} value when applied to
     * each.
     *
     * <p>Note that in most cases, for two instances of class
     * {@code Float}, {@code f1} and {@code f2}, the value
     * of {@code f1.equals(f2)} is {@code true} if and only if
     *
     * <blockquote><pre>
     *   f1.floatValue() == f2.floatValue()
     * </pre></blockquote>
     *
     * <p>also has the value {@code true}. However, there are two exceptions:
     * <ul>
     * <li>If {@code f1} and {@code f2} both represent
     *     {@code Float.NaN}, then the {@code equals} method returns
     *     {@code true}, even though {@code Float.NaN==Float.NaN}
     *     has the value {@code false}.
     * <li>If {@code f1} represents {@code +0.0f} while
     *     {@code f2} represents {@code -0.0f}, or vice
     *     versa, the {@code equal} test has the value
     *     {@code false}, even though {@code 0.0f==-0.0f}
     *     has the value {@code true}.
     * </ul>
     *
     * This definition allows hash tables to operate properly.
     *
     * @param obj the object to be compared
     * @return  {@code true} if the objects are the same;
     *          {@code false} otherwise.
     * @see java.lang.Float#floatToIntBits(float)
     */
    public boolean equals(Object obj) {
        return (obj instanceof Float)
               && (floatToIntBits(((Float)obj).value) == floatToIntBits(value));
    }

Double类

 /**
     * Compares this object against the specified object.  The result
     * is {@code true} if and only if the argument is not
     * {@code null} and is a {@code Double} object that
     * represents a {@code double} that has the same value as the
     * {@code double} represented by this object. For this
     * purpose, two {@code double} values are considered to be
     * the same if and only if the method {@link
     * #doubleToLongBits(double)} returns the identical
     * {@code long} value when applied to each.
     *
     * <p>Note that in most cases, for two instances of class
     * {@code Double}, {@code d1} and {@code d2}, the
     * value of {@code d1.equals(d2)} is {@code true} if and
     * only if
     *
     * <blockquote>
     *  {@code d1.doubleValue() == d2.doubleValue()}
     * </blockquote>
     *
     * <p>also has the value {@code true}. However, there are two
     * exceptions:
     * <ul>
     * <li>If {@code d1} and {@code d2} both represent
     *     {@code Double.NaN}, then the {@code equals} method
     *     returns {@code true}, even though
     *     {@code Double.NaN==Double.NaN} has the value
     *     {@code false}.
     * <li>If {@code d1} represents {@code +0.0} while
     *     {@code d2} represents {@code -0.0}, or vice versa,
     *     the {@code equal} test has the value {@code false},
     *     even though {@code +0.0==-0.0} has the value {@code true}.
     * </ul>
     * This definition allows hash tables to operate properly.
     * @param   obj   the object to compare with.
     * @return  {@code true} if the objects are the same;
     *          {@code false} otherwise.
     * @see java.lang.Double#doubleToLongBits(double)
     */
    public boolean equals(Object obj) {
        return (obj instanceof Double)
               && (doubleToLongBits(((Double)obj).value) ==
                      doubleToLongBits(value));
    }