Java equals的一个坑

public class StringEqualsObject {
    public static void main(String[] args) {

        String name="Tom";
        Person p=new Person(name);
        System.out.println(name.equals(p));
        System.out.println("p.toString():"+name.equals(p.toString()));
        System.out.println(p.equals(name));
    }

}

class Person{
    private String name;

    public Person(String name) {
        super();
        this.name = name;
    }

    @Override
    public String toString() {
        return this.name;
    }
}

Output:

false
p.toString():true
false

原因：
java.lang.String

  /**
     * Compares this string to the specified object.  The result is {@code
     * true} if and only if the argument is not {@code null} and is a {@code
     * String} object that represents the same sequence of characters as this
     * object.
     *
     * @param  anObject
     *         The object to compare this {@code String} against
     *
     * @return  {@code true} if the given object represents a {@code String}
     *          equivalent to this string, {@code false} otherwise
     *
     * @see  #compareTo(String)
     * @see  #equalsIgnoreCase(String)
     */
    public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) {
        String anotherString = (String)anObject;
        int n = count;
        if (n == anotherString.count) {
        char v1[] = value;
        char v2[] = anotherString.value;
        int i = offset;
        int j = anotherString.offset;
        while (n-- != 0) {
            if (v1[i++] != v2[j++])
            return false;
        }
        return true;
        }
    }
    return false;
    }

java.lang.Object

/**
     * Indicates whether some other object is "equal to" this one.
     * <p>
     * The <code>equals</code> method implements an equivalence relation
     * on non-null object references:
     * <ul>
     * <li>It is <i>reflexive</i>: for any non-null reference value
     *     <code>x</code>, <code>x.equals(x)</code> should return
     *     <code>true</code>.
     * <li>It is <i>symmetric</i>: for any non-null reference values
     *     <code>x</code> and <code>y</code>, <code>x.equals(y)</code>
     *     should return <code>true</code> if and only if
     *     <code>y.equals(x)</code> returns <code>true</code>.
     * <li>It is <i>transitive</i>: for any non-null reference values
     *     <code>x</code>, <code>y</code>, and <code>z</code>, if
     *     <code>x.equals(y)</code> returns <code>true</code> and
     *     <code>y.equals(z)</code> returns <code>true</code>, then
     *     <code>x.equals(z)</code> should return <code>true</code>.
     * <li>It is <i>consistent</i>: for any non-null reference values
     *     <code>x</code> and <code>y</code>, multiple invocations of
     *     <tt>x.equals(y)</tt> consistently return <code>true</code>
     *     or consistently return <code>false</code>, provided no
     *     information used in <code>equals</code> comparisons on the
     *     objects is modified.
     * <li>For any non-null reference value <code>x</code>,
     *     <code>x.equals(null)</code> should return <code>false</code>.
     * </ul>
     * <p>
     * The <tt>equals</tt> method for class <code>Object</code> implements 
     * the most discriminating possible equivalence relation on objects; 
     * that is, for any non-null reference values <code>x</code> and
     * <code>y</code>, this method returns <code>true</code> if and only
     * if <code>x</code> and <code>y</code> refer to the same object
     * (<code>x == y</code> has the value <code>true</code>).
     * <p>
     * Note that it is generally necessary to override the <tt>hashCode</tt>
     * method whenever this method is overridden, so as to maintain the
     * general contract for the <tt>hashCode</tt> method, which states
     * that equal objects must have equal hash codes. 
     *
     * @param   obj   the reference object with which to compare.
     * @return  <code>true</code> if this object is the same as the obj
     *          argument; <code>false</code> otherwise.
     * @see     #hashCode()
     * @see     java.util.Hashtable
     */
    public boolean equals(Object obj) {
    return (this == obj);
    }