Java String Concatenation: Rules, Examples, and Unicode Encoding

String Concatenation (joining) is the operation of combining strings. In Java, this operation is performed using the + operator. You can concatenate not only another string but also a value of any other type, which will be automatically converted to a string. How can you determine when the + sign is concatenation and when it is an arithmetic operation?

There are only two rules to remember:

• If one of the operands is of String type, the + operator is used as string concatenation.
• If both operands are numbers, the + operator is used for addition.

In this example, we simply concatenate two strings:

public class StringExample1 {  
    public static void main(String[] args) {  
        String str1 = "world";  
        System.out.println("Hello " + str1);  
        System.out.println("First line\n" + "Second line");  
    }  
}

In the next example, the first System.out.println statement will output "X Y" to the console. We concatenate a char with a String—this is string concatenation. However, in the second System.out.println statement, the output will not be a string, as many expect, but the number 177. Why a number? The char type is a pseudo-integer type and can participate in arithmetic operations. In this case, the character codes are added:

public class StringExample2 {  
    public static void main(String[] args) {  
        char x, y;  
        x = 88;  // Character code for 'X'  
        y = 'Y';  
        System.out.println(x + " " + y); // Outputs X Y  
        System.out.println(x + y); // Outputs 177  
    }  
}

In the next example, the output will be "str=48"—don’t forget about operator precedence. The multiplication operation has a higher precedence than addition:

public class StringExample3 {  
    public static void main(String[] args) {  
        System.out.println("str=" + 4 + 4 * 2);  
    }  
}  

Unicode encoding can also be used in strings, just like in char type variables:

public class StringExample4 {  
    public static void main(String[] args) {  
        // The word "Ukraine" in Unicode encoding  
        System.out.println("\u0423\u043A\u0440\u0430\u0438\u043D\u0430");  
    }  
}

To compare two strings for equality, use the equals method or Objects.equals() (starting from Java 7):

import java.util.Objects;  

public class StringExample5 {  
    public static void main(String[] args) {  
        String str1 = "Hello";  
        String str2 = "Hi";  
        System.out.println(str1.equals(str2));  
        System.out.println(Objects.equals(str1, str2));  
    }  
}

The length of a string is determined using the length() method:

public class StringExample6 {  
    public static void main(String[] args) {  
        String str = "Hello";  
        System.out.println("String length: " + str.length());  
    }  
}