To concatenate two strings str1 and str2 in Java, you can use the + operator or the concat() method.

Here is an example using the + operator:

String str1 = "Hello";
String str2 = "World";
String concatenated = str1 + str2; // "HelloWorld"

Here is an example using the concat() method:

String str1 = "Hello";
String str2 = "World";
String concatenated = str1.concat(str2); // "HelloWorld"

Note that you can also use the += operator to concatenate strings. For example:

String str1 = "Hello";
String str2 = "World";
str1 += str2; // str1 is now "HelloWorld"

You can also use the StringBuilder class to concatenate strings. This is a mutable class that allows you to append strings to the end of a string in a more efficient way than using the + operator or the concat() method. Here is an example:

String str1 = "Hello";
String str2 = "World";
StringBuilder sb = new StringBuilder();
sb.append(str1).append(str2);
String concatenated = sb.toString(); // "HelloWorld"

Finally, you can also use the String.format() method to create a formatted string that includes the concatenation of multiple strings. This method takes a format string as its first argument, which can contain placeholders for the strings you want to concatenate. The placeholders are represented by %s and are replaced by the corresponding arguments passed to the format() method. Here is an example:

String str1 = "Hello";
String str2 = "World";
String concatenated = String.format("%s %s", str1, str2); // "Hello World"