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CS156

Student: ex02

Overriding the toString method, @Override

Part of a series of tutorial articles about a Student class.

Code examples referred to on this page can be found here: https://github.com/ucsb-cs156/student-tutorial/tree/ex02.

Overview

In this example we introduce:

  • overriding the toString method

The example code that supports this lesson is in a repository (“repo” for short) at this link. Note that this link is showing the branch ex02, not the main branch of the repo.

If you have already cloned this repo and are at the command line, you can switch to the ex02 branch with this command:

  • git checkout ex02

Note: this branch uses simple directory structure

Note that the ex01 branch used the full Maven directory structure. By constrast, this branch uses a simple plain directory structure, for contrast.

In ex03 we’ll explain more about the Maven directory structure. For this branch, you can compile and run with with the simple javac/java commands explained in the ex01 branch of the tutorial.

Overriding toString

The problem we are trying to solve is that the code:

        System.out.println("s1 = " + s1); // implicit: r1.toString()
        System.out.println("s2 = " + s2); // implicit: r2.toString()

produces the output:

s1 = Student@38cccef
s2 = Student@64b8f8f4

What we want is something more like this:

s1 = [name: Sample Student, perm: 9999999]
s2 = [name: Chris, perm: 1234567]

To achieve this, we need to override the toString method from java.lang.Object that is producing the values that look like this: Student@38cccef.

Here’s one way to do it:

   @Override
    public String toString() {
        return "[name: " + this.name + ", perm: " + this.perm + "]";
    }

Here, we return literal strings such as "[name: " and ", perm: " concatenated with the value of this.name and this.perm.

Java automatically converts the integer value of this.perm is automatically converted to a String.

Note that it would be incorrect to say that this is done by “a call to the toString method of the this.perm value”; the reason that’s incorrect is that this.perm is of type int, and is therefore not an object in Java. Only objects have methods.

We also see @Override before the declaration of the method. This is an example of an annotation; annotations in Java start with the at sign @, and are used to mark something special about a method or class. In this case, we are telling the Java compiler: “I think I’m overriding a method. If I’m not, please throw an error so that I’ll know.”

This optional, but very much recommended practice. The reason is that if you were to define the following method, without the @Override, you’d get no error message, but it would not do what you want. The reason is that the method is spelled to_string, which is different from toString. Therefore, it would not override the toString method of java.lang.Object, and we’d still get values like Student@38cccef showing up.

    public String to_string() {
        return "[name: " + this.name + ", perm: " + this.perm + "]";
    }

That’s the only change we are going to make for this example.