A Closer Look at Methods and Classes - Using Objects as Parameters

(Page 3 of 12 )

So far we have only been using simple types as parameters to methods. However, it is both correct and common to pass objects to methods. For example, consider the following short program:

// Objects may be passed to methods.
class Test {
  int a, b;
  Test(int i, int j) {
    a = i;
    b = j;
  }
  // return true if o is equal to the invoking object
  boolean equals(Test o) {
    if(o.a == a && o.b == b) return true;
    else return false;
  }
}
class PassOb {
  public static void main(String args[]) {
    Test ob1 = new Test(100, 22);
    Test ob2 = new Test(100, 22);
    Test ob3 = new Test(-1, -1);
    System.out.println("ob1 == ob2: " + ob1.equals(ob2));
    System.out.println("ob1 == ob3: " + ob1.equals(ob3));
  }
}

This program generates the following output:

ob1 == ob2: true
ob1 == ob3: false

As you can see, the equals( ) method inside Test compares two objects for equality and returns the result. That is, it compares the invoking object with the one that it is passed. If they contain the same values, then the method returns true. Otherwise, it returns false. Notice that the parameter o in equals( ) specifies Test as its type. Although Test is a class type created by the program, it is used in just the same way as Java’s built-in types.

One of the most common uses of object parameters involves constructors. Frequently you will want to construct a new object so that it is initially the same as some existing object. To do this, you must define a constructor that takes an object of its class as a parameter. For example, the following version of Box allows one object to initialize another:

// Here, Box allows one object to initialize another.
class Box {
  double width;
  double height;
  double depth;
  // construct clone of an object
  Box(Box ob) { // pass object to constructor
    width = ob.width;
    height = ob.height;
    depth = ob.depth;
  }
  // constructor used when all dimensions specified
  Box(double w, double h, double d) {
    width = w;
    height = h;
    depth = d;
  }
  // constructor used when no dimensions specified
  Box() {
    width = -1;  // use -1 to indicate
    height = -1; // an uninitialized
    depth = -1;  // box
  }
  // constructor used when cube is created
  Box(double len) {
    width = height = depth = len;
  }
  // compute and return volume
  double volume() {
    return width * height * depth;
  }
}
class OverloadCons2 {
  public static void main(String args[]) {
    // create boxes using the various constructors
    Box mybox1 = new Box(10, 20, 15);
    Box mybox2 = new Box();
    Box mycube = new Box(7);
    Box myclone = new Box(mybox1);
    double vol;
    // get volume of first box
    vol = mybox1.volume();
    System.out.println("Volume of mybox1 is " + vol);
    // get volume of second box
    vol = mybox2.volume();
    System.out.println("Volume of mybox2 is " + vol);
    // get volume of cube
    vol = mycube.volume();
    System.out.println("Volume of cube is " + vol);
    // get volume of clone
    vol = myclone.volume();
    System.out.println("Volume of clone is " + vol);
  }
}

As you will see when you begin to create your own classes, providing many forms of constructor methods is usually required to allow objects to be constructed in a convenient and efficient manner.

Next: A Closer Look at Argument Passing >>
 

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This article is excerpted from chapter 7 of Java: the Complete Reference, J2SE 5 Edition, written by Herbert Schildt (McGraw-Hill/Osborne, 2004; ISBN: 0072230738). Check it out at your favorite bookstore. Buy this book now.