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SSD3 - Unit 2Java toString

& EqualsPresentation 3.2.1

Class Website: http://www.bscheele.com/ssd3

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Class Visibility

• We have two choices with respect to the manner in which we declare class visibility:– As public:

public class Student {// Attributes and methods ...

}

– With no explicit visibility:class Professor {

// Attributes and methods …}

• The implications of these two choices are illustrated on the diagrams that follow

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Package A Package B

public class W{ … }

class X { … }

public class Y{ … }

class Z { … }

W.java Y.java

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Package A Package B

public class W{ can create objects of type W and X, but not of Y or Z }

class X { ditto … }

public class Y{ can create objects of type Y and Z, but not of W or X }

class Z { ditto ... }

W.java Y.java

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Package A Package B

import B.*;

public class W{ can create objects of type W, X, and Y, but still not of Z }

class X { ditto … }

public class Y{ can create objects of type Y and Z, but still not of W or X }

class Z { ditto … }

W.java Y.java

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Feature Visibility, Revisited

• We learned about three types of feature visibility early on: public, private, protected

• We hadn't talked about class visibility or packages at that time, and so we oversimplified what these three forms of visibility really entail - we'll revisit that now

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Feature Visibility, Cont.

• Also, if the access type for a given feature is omitted entirely, as in the following example:

public class Person { public String name; private String ssn; protected String address; int age;

then the feature in question is said to have package visibility by default

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Feature Visibility, Cont.

• Here is the effective visibility for a given feature of public class 'A' with respect to a class 'B', depending on where the two classes live:

(Person)

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Feature Visibility, Cont.

hmmm ...

(Person)(Student)

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Feature Visibility, Cont.

hmmm ...

whoa!!!

(Person)(Student)(Course)

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Feature Visibility, Cont.

(assumingimport

of Person)

(Person) (Course)(Student)

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Feature Visibility, cont.

• For same-package classes, the bottom line is that if we don’t explicitly declare something to be private, then it essentially becomes public:

public class Person { // This first attribute is truly private. private String ssn;

// But, all the rest of these attributes are // effectively public from the perspective of

// other classes in the same package. public String name; protected String address; // Exception: subclasses cannot see the next one!

int age;

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Determining the Class that an Object Belongs To

• Another method that all classes inherit from the Object class is:

Class getClass()

• The Class class, in turn , has a method with signature:

String getName()

• Used in combination, we can use these two methods to interrogate an object to find out what class it belongs to

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Determining the Class that an Object Belongs To, cont.

Student s = new Student();Professor p = new Professor();Vector v = new Vector();v.add(s);v.add(p);for (int i = 0; i < v.size(); i++) { // Note that we are not casting the objects here! // We're pulling them out as generic objects. Object o = v.elementAt(i); System.out.println(o.getClass().getName());}

This program produces as output:Student

Professor

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Determining the Class that an Object Belongs To, cont.

• Another way to test whether a given object reference belongs to a particular class is via the instanceof operator – This is a boolean operator which allows us to determine if

some reference variable X is an object/instance of class YStudent x = new Student();

if (x instanceof Professor) … // will evaluate to false

if (x instanceof Person) … // will evaluate to true

– The classname should be the fully qualified name of the class if it belongs to a package, e.g., java.lang.String

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The toString() Method

• We mentioned earlier that all Java classes are descended from the Object class

• One of the features that all classes inherit from Object is a method with signature: public String toString();

• As inherited, however, the method may not prove to be very useful, as the following example illustrates (cont.)

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toString(), cont.

Student s1 = new Student(); s1.setName("Harvey"); System.out.println(s1); // Prints out an object ID … not too useful!

Student@71f71130

• It is a good idea to explicitly override the toString() method for any class that you invent

public String toString() { return getName(); }

// Prints "Fred" (or whatever). System.out.println(s1);

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Testing for Equality

• We've seen the use of a double equal sign (==) to test for equality of two values; e.g.,int x = 3;int y = 4;if (x == y) do something …

• We can also use == to test the "equality" of two objects:Person p1 = new Person("Joe");Person p2 = new Person("Mary");if (p1 == p2) do something ...– What does "equality" mean in the case of two object

references?– As we saw from our discussion of String as objects, the ==

operator tests to see if two references are referring to the same object

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Testing for Equality, cont.

public class Person {private String name;

// Constructor.public Person(String n) {

name = n;}

}

// Client code:Person p1 = new Person("Joe");Person p2 = p1; // Second handle on SAME object.Person p3 = new Person("Joe"); // New object, same NAME.

if (p1 == p2) System.out.println("p1 equals p2");if (p1 == p3) System.out.println("p1 equals p3");

Prints out: p1 equals p2

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The equals() Method

• Another of the features that all classes inherit from Object is a method with signature: public boolean equals();

• This method is used to test the "equality" of two objects in a different way: namely, based on whatever criteria that we establish by overriding the method – Let's look at an example

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The equals() method, cont.

public class Person {private String name;

// Constructor.public Person(String n) {

name = n;}

// Overridden from Object.public boolean equals(Object o) { boolean answer = false; try {

Person p = (Person) o; // Note cast.if (p.getName().equals(name)) answer = true;else answer = false;

} catch (ClassCastException e) {

answer = false; }

return answer;}

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The equals() method, cont.

public String getName() {return name;

}}

// Client code.

Person p1 = new Person("Joe");Person p2 = new Person("Mary");Person p3 = new Person("Joe"); // Different object, same name.Object o = new Object();

System.out.println("Does p1 equal p2? " + p1.equals(p2));System.out.println("Does p1 equal p3? " + p1.equals(p3));System.out.println("Does p1 equal o? " + p1.equals(o));

Produces as output: Does p1 equal p2? falseDoes p1 equal p3? trueDoes p1 equal o? false

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Reading from the Command Line

• Now that we’ve learned about Java Strings and arrays, we can appreciate how to pass data into a Java program when invoking it from the command line

• When we invoke a program, we can type data after the name of the program on the command line; e.g.

java Simple ABC 123

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Command Line Input, cont.

• Such data gets handed to the main() method as a String array called ‘args’ (or whatever else we wish to name it), as indicated by the main() method's argument signature:

public static void main(String[] args) { … }

• Inside the main() method, we can do with 'args' whatever we’d do with any other array– E.g, determine its length, manipulate individual String

items within the array, and so forth

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Command Line Input, cont.

public class FruitExample {

// If this program is run from the

// command line as follows:

//

// java FruitExample apple banana cherry

//

// then the args array will be automatically

// initialized with THREE String values

// "apple", "banana", and "cherry", which

// will be stored in array 'cells' args[0],

// args[1], and args[2], respectively.

public static void main(String[] args) {

// Let's print out a few things.

System.out.println("The args array contains "

+ args.length + " entries.");

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Command Line Input, cont.

// Only execute this next block of code if // the array isn't empty. if (args.length > 0) { int i = args.length - 1; System.out.println("The last entry is: " + args[i]);

System.out.println("It is" + args[i].length() +

" characters long."); }

// etc. }}

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Command Line Input, cont.

• When this program is run from the command line as: java FruitExample apple banana cherry

it produces the following output:

The args array contains 3 entries.

The last array entry is: cherry

It is 6 characters long.

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Questions?