[Java] StringとStringBuilderのややこしいところ

StringとStringBuilderの比較

System.out.println(new StringBuilder("abc")); //=> abc
System.out.println(new StringBuilder("abc").toString()); //=> abc

System.out.println("abc".equals(new StringBuilder("abc"))); //=> false
System.out.println("abc".equals(new StringBuilder("abc").toString())); //=> true

StringとStringBuilderは継承や実装の関係になく、Stringクラスのequalsメソッドが以下のようにオーバーライドされているため。

/**
     * 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.
     *
     * <p>For finer-grained String comparison, refer to
     * {@link java.text.Collator}.
     *
     * @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 aString = (String)anObject;
            if (coder() == aString.coder()) {
                return isLatin1() ? StringLatin1.equals(value, aString.value)
                                  : StringUTF16.equals(value, aString.value);
            }
        }
        return false;
    }