Today. Book-keeping. Inheritance. Subscribe to sipb-iap-java-students. Slides and code at Interfaces.

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1 Today Book-keeping Inheritance Subscribe to sipb-iap-java-students Interfaces Slides and code at The Object class Problem set 1 released 1 2 So far... Inheritance Basic objects, OOP More refined type of class Arrays Some Files are TextFiles Strings Some TextFiles are JavaSourceFiles May only extend one class 3 4

2 Inheritance, cont d Derived class objects can do everything that the base class can File has a getfilename() method TextFile has the same method Inheritance, an example public class File { public String getfilename() { public class TextFile extends File { public String gettext() { 5 6 Compile-time vs Run-time Variables of type File can hold File, or any derived type At compile-time, we say it s a File, but we can store a TextFile in it Because a TextFile is a kind of File and can do everything a File can Compile-time vs Run-time, an example File f; f = new TextFile("Test.java"); String s = f.getfilename(); // Okay // File doesn t have this // method; compile-time error s = f.gettext(); TextFile tf; // Not every File is a TextFile; // compile-time error tf = new File("Test2.java"); tf = new TextFile("Test2.java"); 7 8

3 Typecasting Sometimes you know the run-time type (NewType)variable coerces variable s type to NewType Compile-time error if NewType isn t derived from variable s compile-time type Typecasting, an example File f; f = new TextFile("file.txt"); String s = f.getfilename(); TextFile tf; tf = (TextFile)f; s = tf.gettext(); Can result in run-time error 9 10 Typecasting, an example cont d instanceof File f; f = new JavaClassFile(); TextFile tf = (TextFile)f; // This is a run-time error, // since JavaClassFiles aren t // TextFiles Might be useful to find the run-time type of a variable instanceof operator does this test 11 12

4 instanceof, an example File f; // Assume it comes from somewhere, // don t know runtime type if (f instanceof TextFile) { TextFile tf = (TextFile)f; String s = tf.gettext(); instanceof, an example cont d File f = new JavaSourceFile; if (f instanceof TextFile) { TextFile tf = (TextFile)f; String s = tf.gettext(); // Still works fine, because // JavaSourceFiles are a kind // of TextFile Overriding methods JavaSourceFile could define its own get- FileName method Would be called in place of File s method Overriding methods, an example public class File { public String getfilename() { public class JavaSourceFile extends TextFile { public String getfilename() { 15 16

5 Overriding methods, an example cont d Calling superclass code File f = new File("foo"); String s = f.getfilename(); // Runtime type of f is File, so // calls File s getfilename() f = new JavaSourceFile("bar.java"); s = f.getfilename(); // Runtime type of f is JavaSourceFile, // so JavaSourceFile s getfilename() is // called, even though compile-time // type is File Overriding methods might want to call on the method they re overriding Extremely common in constructors Also common elsewhere Use super as an object Calling superclass code, an example Constructors and super // In JavaSourceFile class... public JavaSourceFile(String fn) { super(fn); // Calls constructor classname = null; public void getfilename() { if (classname!= null) return classname + ".java"; return super.getfilename(); Superclass s constructor is always called as first thing in derived class s constructor If your first statement is a super(...) this(...) call, great Otherwise, Java inserts super(); for you or 19 20

6 Overloading methods Overloading methods, an example Method with same name, different parameters Can also overload constructors public JavaSourceFile(String filename, File classfile) { super(filename); this.classfile = classfile; Should do similar things public JavaSourceFile(String filename) { this(filename, null); Might provide default parameters calls an- Within a constructor, this(...) other constructor Overloading methods, traps Access control, reprise When overridden methods have different arguments than in superclass, doesn t actually override Check that what you re overriding has the correct signature. public: anyone, anywhere protected: same class, package or derived class Default: same class or derived class private: same class only 23 24

7 Final methods Might want a method to not be able to be overridden Use final keyword Final methods, an example // Within File class... final public int getunixpermissions() { // Call into the OS; don t let derived // classes lie by overriding us Rarely used in practice Abstract methods Abstract classes Require that a method exist Provide no implementation in base class All derived classes must either provide implementation or be abstract A class is abstract if it has at least one abstract method No objects can have a run-time type of an abstract class Variables may have that compile-time type, however 27 28

8 Abstract classes, an example public abstract class Shape { abstract int area(); public class Rect extends Shape { private int width, height; Class derivation public class Square extends Rect { public Square(int sidelen) { super(sidelen, sidelen); public Rect(int w, int h) { width = w; height = h; public int area() { return width * height; Putting it together Class modifiers Shape shape1, shape2; shape1 = new Square(5); shape2 = new Rect(4, 6); if (shape1.area() > shape2.area()) { Keywords may appear before the class declaration public: Can be used anywhere, not just current package abstract: May not be instantiated final: May not be subclassed 31 32

9 Interfaces Like a class with only public abstract methods A class may declare that it implements one or more interfaces Interfaces, an example public interface Colored { Color getcolor(); public class ColoredRect extends Rect implements Colored { private Color color; public ColoredRect(int w, int h, Color c) { super(w, h); color = c; public Color getcolor() { return color; Object class Object equality, reprise Every class implicitly extends java.lang.object Thus, there are some methods that all objects have Object.equals() tests if two (different) objects are the same Default implementation is just == Especially important when using inheritance, as you might inherit an equals from superclass Which may not check everything that your class needs to Fine to call super.equals(o) 35 36

10 Object equality, one example String conversion // In class Rect... public boolean equals(object o) { if (!o instanceof Rect) return false; Rect r = (Rect)o; if (this.width!= r.width) return false; if (this.height!= r.height) return false; return true; Object.toString() returns a String representation of an object Very useful for debugging Less useful if you forget to override superclass tostring and you get mislead String conversion, an example Hash codes // In class Rect... public String tostring() { return "Rect " + width + "x" + height; // In class Square... public String tostring() { return "Square " + width; Provide a mostly-unique integer for each object equals objects must have the same hashcode At very worst, hashcode could just always return the same value Default implementation works Sometimes you can do better 39 40

11 Hash codes, an example In summary // In class Rect... // (A bit silly, as the default // implementation is probably better) public int hashcode() { return width + 17*height; Use inheritance to model complex systems Interfaces to group across inheritance trees Methods inherited from Object class Tomorrow Exceptions Collections 43