String Concatenation in Java

In Java, you can concatenate multiple strings with StringBuilder, StringBuffer, String's concat(String) method or + operator (ordered by performance preference)

Using StringBuilder

You can use StringBuilder's append or insert method to concatenate strings. They are overloaded to accept any data type. While append method always adds the string at the end of the builder, the insert method adds the string at a specified index

Instances of StringBuilder are not safe to be used by multiple threads. In that case, consider using StringBuffer

@Test
public void givenMultipleDataTypes_whenConcatByStringBuilder_thenSuccess() {  
    StringBuilder sb = new StringBuilder();
    sb.append("Hello ");
    sb.append("Koding ");
    sb.append(2015);
    String result = sb.toString();

    assertThat(result).isEqualTo("Hello Koding 2015");
}

Using StringBuffer

StringBuffer has the API compatible with StringBuilder, so you can also use append or insert methods of StringBuffer to concatenate strings

StringBuffer is designed to be used in multiple threads environment

@Test
public void givenMultipleDataTypes_whenConcatByStringBuilder_thenSuccess() {  
    StringBuffer sb = new StringBuffer();
    sb.append("Hello ");
    sb.append("Koding ");
    sb.append(2015);
    String result = sb.toString();

    assertThat(result).isEqualTo("Hello Koding 2015");
}

Using String's concat(String) method

The Java String class provides the concat(String) method to concatenate the specified string to the end of the caller string

The concat method only accept String data type, so try converting the argument to String before passing to the method

@Test
public void givenMultipleDataTypes_whenConcatByStringConcat_thenSuccess() {  
    int foundedYear = 2015;
    String result = "Hello ".concat("Koding ").concat(Integer.toString(foundedYear));

    assertThat(result).isEqualTo("Hello Koding 2015");
}

Using + operator

The Java + operator will try to convert the input parameters to String before concatenation. It offers convenient typing but with slowest performance as a new String object will be created on every operations

@Test
public void givenMultipleDataTypes_whenConcatByPlus_thenSuccess() {  
    int foundedYear = 2015;
    String result = "Hello " + "Koding " + foundedYear;

    assertThat(result).isEqualTo("Hello Koding 2015");
}

Conclusion

In this tutorial, we learned various methods to concatenate strings in Java. While String's concat and + operator are used in simple cases, StringBuilder is recommended to be used when you want to care about the performance. The full source code can be found as below