Constructors, this, and Your First Method

A constructor builds an object fully formed at new. this is how you tell a field apart from a parameter that shares its name — get it wrong, and the field stays empty.

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Setting every field by hand after new is tedious and easy to forget — miss one line and a field is left empty. A constructor fixes that. It's a special block that runs when an object is created, so the object arrives fully built. While you're adding behavior, give the class a method too — an operation that belongs to the object:

class TestResult {
    String name;
    String status;
    int durationMs;

    TestResult(String name, String status, int durationMs) {
        this.name = name;
        this.status = status;
        this.durationMs = durationMs;
    }

    boolean isPassed() {
        return status.equals("PASS");
    }
}

TestResult login  = new TestResult("LoginTest", "PASS", 240);
TestResult search = new TestResult("SearchTest", "FAIL", 512);
System.out.println(login.name + " passed? " + login.isPassed());
System.out.println(search.name + " passed? " + search.isPassed());

LoginTest passed? true
SearchTest passed? false

this — one keyword, one reason it exists

The constructor has the same name as the class and no return type, and it runs once, at new. Inside it you see this, the keyword that means this particular object. So this.name = name reads as: set this object's name field to the name that was passed in. The reason this is needed here is that the parameter and the field share the name name — this.name says which one you mean. Get this wrong and you get a real bug, one worth its own article later in this chapter.

isPassed() is a method — behavior that belongs to the object. It uses the object's own status field, and notice it compares with .equals, exactly as Chapter 6 taught. A TestResult now carries its data and knows how to answer a question about itself.