Today we are going to focus on how you can compare Strings in Java. By comparing, in this case, we mean to check if their values are equal.
In the previous example, we talked generally about the Java String Class. We stated that Java lets you initialize a String like a primitive, and use the ‘+’ operator like you would in a primitive, but here used to concatenate Strings together.
The similarities stop here. A String is in no way a primitive type. It is a classic Java Object. So comparing a String is no different from comparing any other Java Object.
1. The ‘==’ operator
It is strongly (and correctly) advised that you should never use ‘==’ to compare any two Objects. Let’s see an example
Equals Operator
//Objects a and b are of the same non primitive type
if(a==b)
System.out.println("The to objects are equal");
In the above example we are comparing two references, not objects. If the two references are equal, it simply means that they are pointing to the same Object instance. Consequently, a and b are equal, as they are exactly the same object.
But in most cases, you have two different discrete object of the same type that have equal contents. And it is the contents that matter in an equality check. This is where you use the equals method.
2. Java Compare Strings – Using equals
equals is a member of the Object class, so any class in Java can override it an create its own customized equality check.
Here is how you can use it in Strings:
Equals
String a = "Java is great!";
String b = "Java is great!";
if(a.equals(b))
System.out.println("The strings are equal");
The output of the above is:
The strings are equal
Due to String pooling that we’ve talked about in the previous example, the ‘==’ operator also works:
Equals Operator
String a = "Java is great!";
String b = "Java is great!";
if(a == b)
System.out.println("The strings are equal");
The output of the above is:
The strings are equal
That’s because literals with the same value are the same object exactly.
Now, take a look at this :
Equals With object
String a = "Java is great!";
String b = new String("Java is great!");
if(a == b)
System.out.println("The strings are equale");
else
System.out.println("The two strings are not the same Object");
if(a.equals(b))
System.out.println("But they hold the same string");
The output of the above is:
The two strings are not the same Object
But they hold the same string
So you see why it is important to use equals for string comparison.
Let’s see the following snippet:
Equals With different Case
String a = "Java is great!";
String b = "Java Is Great!";
if (a.compareTo(b) == 0)
System.out.println("Strings are equal");
else
System.out.println("Strings are NOT equal");
The above prints out :
Strings are NOT equal
This is because Strings in Java are case-sensitive, no matter the platform you are working on.
3. Using equalsIgnoreCase
If you don’t want to make case sensitive comparison, aka for you, the strings “abcd” and “AbCD” are the equal, then you can use equalsIgnoreCase:
equalsIngoreCase
String a = "Java is great!";
String b = "Java Is Great!";
if (a.compareTo(b) == 0)
System.out.println("Strings are equal");
else
System.out.println("Strings are NOT equal");
This will print out:
Strings are equal
4. Using compareTo
This is useful to make lexicographical comparison between two strings. This generates the Unicode value of each character in the string and compares with the Unicode value of other string.
CompareTo
String a = "Java is great!";
String b = "Java Is Great!";
if (a.compareTo(b) == 0)
System.out.println("Strings are equal");
else
System.out.println("Strings are NOT equal");
System.out.println("a>b:"+a.compareTo(b));
In the example above, it does character by character comparison. As soon as it reaches the character “I”, the comparison ends. The Unicode value for ‘i’ is 105 while ‘I’ is 73. So result is returned as 32 and no further comparison proceeds. Result of running this program is indicated below
Strings are NOT equal a>b:32
5. Using contentEquals
To understand contentEquals, we have to understand CharSequence interface. This was introduced after the concrete implementation of String. This method allows a String to be compared with any other implementations of CharSequence such as StringBuilder and StringBuffer. It does a character by character comparison and if any of the characters do not match, it returns false.
CompareTo
String a = "Java is great!";
String b = "Java Is Great!";
if (a.contentEquals(new StringBuilder(b)))
System.out.println("Strings are equal");
else
System.out.println("Strings are NOT equal");
This will print out:
Strings are NOT equal
6. Using comparison with literals
As we’ve said in the previous tutorial, literals are implemented as instances of String. We can compare literals like they were string objects.
Let’s see:
Literal Comparison-1
String a = "abc";
String b = "aBc";
System.out.println(a.equals("abc"));
System.out.println(b.equalsIgnoreCase("abc"));
System.out.println(a.compareTo("abc"));
System.out.println(b.contentEquals("abc"));
You can also do it like so:
Literal Comparison-2
String a = "abc";
String b = "aBc";
System.out.println("abc".equals(a));
System.out.println("abc".equalsIgnoreCase(b));
System.out.println("abc".compareTo(a));
System.out.println("abc".contentEquals(b));
This way you can avoid NullPointerException. But be careful because the absence of NullPointerException doesn’t make the program correct.
This was an example on how to compare Strings in Java.
7. Download the Source code
That was an example on Java Compare Strings.
You can download the full source code of this example here: Java Compare Strings Example
Last updated on Sept. 23, 2019
