Espresso: A Concentrated Introduction to Java

Standard Object Methods

Summary: We consider the standard methods that all (or most) objects provide.

Prerequisites: Basics of Java.


Standard Methods

In Java, all template classes provide a variety of standard methods. By standard method, I mean that it is a standard that you include the method. You have already encountered one such method, toString.

Why does Java have standard methods? Because programmers benefit from knowing that certain methods are there. For example, you can easily convert any object to a string with the toString method, and you don't even have to bother to look it up. Similarly, you can compare any two objects in the same class for equality with the equals method.

Java has two groups of standard methods. Some must be there for every class. These methods include toString and equals. Others you may choose whether or not to include, and you specify in an interesting way that you have included them. These methods include clone and compareTo.

What happens if you don't write the standard methods? Java supplies its own, and they don't always work the way you would expect them to work.

Converting to Human-Readable Form with the toString Method

One of the simplest methods that most classes provide is the toString method, which has the signature

public String toString()

As the name of the method suggests, this method converts the object to a string. Why would we want to convert an object to a string? Most typically, so that we can print the object (to the screen, to a file).

The toString method is so fundamental, that Java uses it implicitly in a number of cases. For example, whenever you try to print an object, Java calls the toString method, even if you haven't explicitly listed it in your call. Similarly, if you try too concatenate another object with a string using the + operation, Java converts the object to a string using the toString method.

What happens when you don't write the toString method? Java supplies a default version that prints the class of the object followed by the address of the object in memory. (Yes, it's a strange default, but it's about as good as anything.)

Comparing Values with the equals Method

Another simple standard method is equals, which you use to determine whether one object is naturally the same as another object. You first have a responsibility to determine what naturally means. For example, are two decimal numbers the same if they are not precise equal? Are two fractions the same if one is simplified and one is not? Are two vectors in two-space the same if one has an angle of 360 degrees more than the other? Are two vectors in two-space the same if they have the same radius, and their angles differ by a very small amount (less than 1/1000 of a degree)?

The standard signature for the equals method is

public boolean equals(Object other)

Note that it is somewhat difficult to compare a specific object to a general object. So, what do you do? My standard solution is to provide a second form of equals that takes the specific kind of object as a parameter, and then, in the first equals, call the second while casting the object to that type. (Recall that you cast a value to another type by prefacing the name of the value with the name of the type in parentheses.)

For example, in the Fraction class, we might write

public boolean equals(Object other)
  return this.equals((Fraction) other);
} // equals(Object)
public boolean equals(Fraction other)
  return this.numerator.equals(other.numerator)
         && this.denominator.equals(other.denominator);
} // equals(Fraction)

What happens if the cast fails? Java throws a ClassCastException. How do you then write the equals(Object) method? There are two choices. You can catch that exception, or you can check whether the cast is safe with the instanceof operation.

Using the prior strategy, you might write

public boolean equals(Object other)
  try {
    return this.equals((Fraction) other);
  catch (ClassCastException cce) {
    return false; // If other isn't a fraction, it's not equal.
} // equals(Object)

Using the latter strategy, you might write

public boolean equals(Object other)
  if (other instanceof Fraction) {
    return this.equals((Fraction) other);
  else {
    return false;
} // equals(Object)

Of course, you should express that more concisely as

public booleans equals(Object other)
  return (other instanceof Fraction) 
         && this.equals((Fraction) other);
} // equals(Object)

If you don't bother to write equals, Java provides a default version that returns true only if the two values share the same memory locations.

Comparing Objects with compareTo

The equals method provides the simplest form of comparison. At times, we need more complex comparison, such as when we want to put a sequence of values in order from smallest to largest. For such situations, objects may (but need not) provide the compareTo method, which you may have used with some of the standard classes, such as java.math.BigInteger.

This method has the signature

public int compareTo(Class other)

where Class is typically the class you're defining. For example, in the Fraction class, we would write

public boolean compareTo(Fraction other)

The this.compareTo(other) method returns

How should you decide if one object naturally precedes another? That's up to you. What if you can't choose such a relationship? Then you shouldn't bother implementing compareTo. What if there are many possible relationships, as in the case of of students, who you might compare by name, by age, by student ID, by height, by GPA, or by something completely different? Then yoou need to pick one as the default or choose no default. In a subsequent reading, we'll consider how to handle multiple comparisons.

The ordering given by compareTo should be transitive and reflexive. When we say that it is transitive, we mean that if a.compareTo(b) returns a negative number and b.compareTo(c) returns a negative number, then a.compareTo(c) should also return a negative number. (We can say something similar when it reutrns positive numbers.) When we say that it is reflexive, we mean that if a.compareTo(b) returns a negative number, then b.compareTo(a) should return a positive number (and vice versa).

Because this standard method is not always implementable (that is, there is sometimes no natural ordering), you need not include it. If you do you should add the following line to the header of your class implements Comparable<Class> You must also import java.util.Comparable.

For example,

import java.util.Comparable;

public class Fraction
  implements Comparable<Fraction>

Note that for the compareTo method, you need not follow the "two method" strategy that you had to use in equals. Only one compareTo, which compares to objects in the same class, is all that is necessary.

Copying Objects with the clone Method

At times, you have one copy of an object and you need another copy. For example, you may have created a StringBuffer and want to keep the original and make a copy that you will modify. To support such situations, Java encourages you to provide a method called clone.

The signature of this method is

public Object clone()

You may find it strange that clone returns an Object rather than explicitly returning a member of the specified class. This form of return was all that was supported in an early version of Java (that is, there was no way to have multiple methods with the same name and parameter types, but different return types), and it seems to have been retained.

Because clone returns an object, you need to cast the return value. For example, the Java compiler will complain about

Fraction frac2 = frac1.clone();

and insist that you instead write

Fraction frac2 = (Fraction) frac1.clone();

Although clone is standard, it is also optional. If you supply the method, you should indicate that your class implements Cloneable. For example,

public class Fraction
  implements Cloneable, Comparable<Fraction>

The Mysterious hashCode Method

The last of the standard methods is somewhat strange. The hashCode method returns some integer that represents this object. What integer should you return? It's up to you. The two general rules are that

Of course, it is impossible to guarantee that unequal objects have different numbers, since there are typically more objects than there are integers. You should simply try to give unequal objects different numbers.

Why does Java include the hashCode method as a standard method? That method is very useful for hash tables, which we will consider later in the semester. (The designers of Java made some strange decisions as to what to make default. Some folks find this one of the stranger ones.)

Do you have to write the hashCode method? It's not a bad idea. The Java standard suggests that if you write equals, then you must write hashCode.

What is the default behavior of hashCode? By default hashCode returns some value computed from the memory location of the object. Hence, two equal values are unlikely to have the same hash code.


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Samuel A. Rebelsky