Objects

Class instances exist within a hierarchy. At the top, a superclass exists: this is Object. We can treat any instance as an object.

The Objects `class`

Another class, Objects, offers many helpful utility methods. These allow us to validate objects (with requireNonNull) or compute values based on them (with hash).

`RequireNonNull`

Let us begin with this method. We pass an Object to requireNonNull—it does nothing unless the object is null. If null, it throws an Exception.

Note This method is used for parameter validation, as in constructor bodies. Please see the next example.

import java.util.Objects;

public class Program {
    public static void main(String[] args) {

        // Ensure an object is not null.
        String value = null;
        Objects.requireNonNull(value);
    }
}
Exception in thread "main" java.lang.NullPointerException
    at java.util.Objects.requireNonNull(Unknown Source)
    at program.Program.main(Program.java:11)

Validate constructor

Here we use Objects.requireNonNull to validate a constructor on a test class. The String argument must not have a null value. It is required.

Note In main we test the constructor. When called with a String like "Andrew" it works with no error. But null causes an Exception.

import java.util.Objects;

class Test {
    public Test(String name) {
        Objects.requireNonNull(name, "name cannot be null");
    }
}

public class Program {
    public static void main(String[] args) {

        // This is safe.
        Test t = new Test("Andrew");

        // This will cause an exception.
        Test t2 = new Test(null);
    }
}
Exception in thread "main" java.lang.NullPointerException: name cannot be null
    at java.util.Objects.requireNonNull(Unknown Source)
    at program.Test.<init>(Program.java:8)
    at program.Program.main(Program.java:18)

`Objects.hash`

This method computes a hash code for one or more objects. We pass values directly as arguments. It can be used to implement the hashCode method.

Here We implement hashCode on the Test class. The hash for Test is based on two parameters: the X and Y fields.

Tip This is a convenience method. When we have just one object to base a hash code on, using hashCode directly is clearer.

import java.util.Objects;

class Test {

    int x;
    int y;

    public Test(int x, int y) {
        this.x = x;
        this.y = y;
    }

    @Override
    public int hashCode() {
        return Objects.hash(this.x, this.y);
    }
}

public class Program {
    public static void main(String[] args) {

        // Get hash code for these objects.
        Test t = new Test(1, 10);
        System.out.println(t.hashCode());
        Test t2 = new Test(2, 20);
        System.out.println(t2.hashCode());
    }
}1002
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An abstraction

This abstract model gives programs the powerful capability of generalization. Any instance, now, may be treated an object.

Usually, we access methods defined directly on a class. But methods from the Object class are universal, and help in many program parts.

Helpers

With the Objects class, we access methods that act on many objects. Some methods, like requireNonNull, are helper methods, ones that make a common pattern easier to implement.