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6.4 Comparing objects

6.4 Comparing objects. Comparing Strings. The general subject of comparing objects, or object references, can be introduced concretely with strings. Recall that String is a class, and so strings themselves are instances of this class. The String class implements a method called equals().

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6.4 Comparing objects

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  1. 6.4 Comparing objects

  2. Comparing Strings • The general subject of comparing objects, or object references, can be introduced concretely with strings. • Recall that String is a class, and so strings themselves are instances of this class. • The String class implements a method called equals(). • This is the method that checks to see whether the contents of two different string objects are the same.

  3. Comparing Equal Strings • Let two strings be declared and initialized as follows: • String myString = “Hello World”; • String yourString = “Hello World”; • The situation can be diagrammed in this way:

  4. Comparing Equal Strings • Two separate objects exist, containing the same sequence of characters. • Here is an if statement containing a call to the equals() method. • In this situation the condition evaluates to true. • if(myString.equals(yourString))

  5. Comparing Unequal Strings • If these string values were assigned to the two references, however, the equals() method would return false: • myString = “Tra-la-la”; • yourString = “La-de-dah”;

  6. Complication when comparing Strings • The complication comes when you consider the use of the operator ==. • This tests for equality of reference, not equality of contents. • In other words, it tests to see whether the objects themselves are the same, not whether their contents are the same. • I consider this to be a major pitfall for new programmers, so make sure to understand this.

  7. Comparing Strings with == • Let the two strings now be declared and initialized as follows: • String myString = “Hello World”; • String yourString = myString; • if(myString == yourString) • The situation can be diagrammed in this way:

  8. Comparing Strings with ==, cont. • Only one object exists, with two references to it. • Here is an if statement containing a test of equality of reference. • In this situation the return value would be true. • if(myString == yourString)

  9. Other methods for comparison • There are other methods in the String class that support various kinds of comparisons. • Among them are compareTo() and equalsIgnoreCase(). • The first of these allows you to compare the contents of strings to see which might come first alphabetically. • The second method allows you to check for equality of contents of strings where small and capital letters don’t make a difference. This is done by returning a boolean result.

  10. Other methods for comparison, cont. • This can be useful when taking in input from users. • Complete information on each of these methods can be found in the Java API documentation.

  11. Comparing Equality for Objects • The same observations that were made for comparing equality of strings can be made for comparing equality of objects in general. • Suppose you have the following two declarations and initializations: • Point2D.Double myPoint = new Point2D.Double(10, 20); • Point2D.Double yourPoint = new Point2D.Double(30, 40);

  12. Comparing Equality for Objects, cont. • The contents are different and the objects are different. Both the equals() method and the == would return false. • Suppose instead that the initializations were as follows: • Point2D.Double myPoint = new Point2D.Double(10, 20); • Point2D.Double yourPoint = new Point2D.Double(10, 20);

  13. Comparing Equality for Objects, cont. • Now there are two different objects with the same contents. • The equals() method would return true while the == would return false. • Finally consider this case: • Point2D.Double myPoint = new Point2D.Double(10, 20); • Point2D.Double yourPoint = myPoint;

  14. Comparing Equality for Objects, cont. • Now both equals() and == would return true. • The references refer to the same object, and by definition the contents of the object referred to by the references are the same.

  15. equals() and == for your own classes • This description of equals() and == applies to system supplied classes. • However, beware: equals() will not work this way with classes you have written. • For example, suppose I have the following objects, created using my own class: • Shampoo myShampoo = new Shampoo(); • Shampoo yourShampoo = new Shampoo();

  16. equals() and == for your own classes, cont. • No matter what the contents of these objects might be, the equals() method will work exactly like the ==. • In other words, for your classes it is possible to call the equals() method, but it only tests for equality of reference, not equality of contents.

  17. equals() and == for your own classes, cont. • Your class inherits the method equals() from a class above it in the hierarchy. • However, the inherited implementation does not work in the desired way. • How to write an equals() method will be covered in a future unit.

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