Have you ever wanted to get an in-depth understanding of methods and classes in Java? Look no further. 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).
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 Testcompares 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 Testis 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.