How to compare strings in Java?
In my program, I used the == operator to compare strings. But I came across a bug, and when replacing == with equals, it disappeared.
Should the == operator be avoided? When can it be used and when not? What's the difference?
2 answers
The == operator compares links.
The equals method compares the values.
Therefore, if you want to compare strings for equality, you should use equals.
However, in some cases, strings are guaranteed to be represented by the same object thanks to the string pool (string interning). These cases are explicitly described in the Java language specification.
The == operator is used to check that two strings point to the same object.
// Эти строки имеют одно и тоже же значение
new String("test").equals("test") // --> true
// ...но это разные объекты
new String("test") == "test" // --> false
// ...эти строки тоже разные объекты
new String("test") == new String("test") // --> false
// ...но эти строки указывают на один и тот же объект,
// потому что компилятор добавляет все литералы в пул.
"test" == "test" // --> true
// Конкатенация литералов тоже происходит на стадии компиляции,
// поэтому они указывают на один объект
"test" == "te" + "st" // --> true
// но вызов substring() происходит во время выполнения,
// в результате получаются разные объекты.
"test" == "!test".substring(1) // --> false
// Строки из пула могут быть получены с помощью вызова intern().
"test" == "!test".substring(1).intern() // --> true
It should be noted that == is noticeably faster than equals (comparing a reference instead of calling a method and a character-by-character comparison if the strings are of different lengths), so if you are working with strings from a pool (either the system or your own), replacing equals with == can lead to a noticeable acceleration. But this happens very rarely.
Beware of calling equals on null! The == operator perfectly compares strings if one or more of them is equal to null, but calling the equals method on a string equal to null will result in an exception.
To compare strings that can be equal to null, you can use the following method:
public static boolean equals(String str1, String str2) {
return str1 == null ? str2 == null : str1.equals(str2);
}
It is present in some third-party libraries, such as Apache Commons.
If you use modern development environments, they will warn you if you try to compare strings using the == operator. Always pay attention to such things warnings.
In short, == compares references to an object, if the references point to the same object, then it is true, otherwise false, in the case of primitive types == compares values.
equals () it is used in String so, it takes and compares each String character by character, but this is only with String, if you take the other objects (you created objects Яблоко and Груша), while these classes do not have the equals method, then it is like == compares references to an object, if it is the same object, then tru otherwise false.
In String, the equals () method is written, which compares character-by-character, so you need to use equals () {[15 withString]}