ADSA: Generics/ Advanced Data Structures and Algorithms Objective –to describe basic forms of generic classes, interfaces, and methods for searching and sorting Semester 2, Generic Classes and Methods

ADSA: Generics/2 2 Contents 1. The Object Superclass 2. Generalizing Sequential Search 3. General Collections 4. Better Collections with Generics 5. Generic Interfaces 6. The Comparable Interface 7. Generic Methods 8. Generic Searching / Sorting

ADSA: Generics/ The Object Superclass Every class is a subclass (or sub-subclass, sub-sub-subclass, etc) of the Object class.

ADSA: Generics/2 4 Object Class Methods Boolean equals(Object obj) –is obj “equal to” this object? String toString() –returns a string representation of this object Object o = new Object(); o.equals(obj); String s = o.toString();

ADSA: Generics/ Generalizing Sequential Search Our seqSearch() method from Part (1) can only search int[] arrays for an integer target –the code is shown again on the next slide A generalized version uses an Object array and an Object target –see slide 7

ADSA: Generics/2 6 Sequential Search with int public static int seqSearch(int[] arr, int first, int last, int target) { // scan elements in the range // first <= i < last; test for a match for (int i = first; i < last; i++) if (arr[i] == target) return i; // match not found return -1; }

ADSA: Generics/2 7 Sequential Search with Object public static int seqSearch(Object[] arr, int first, int last, Object target) { // scan elements in the range // first <= i < last; test for a match for (int i = first; i < last; i++) if (arr[i].equals(target)) return i; // match not found return -1; } continued

ADSA: Generics/2 8 This generalized search method can deal with arrays of any subclass (or sub-subclass, etc) of Object –now seqSearch() can search polymorphic data structures Object[] arr = new Object[3]; arr[0] = new Date(); arr[1] = new HourlyEmployee("andrew"); arr[2] = new Integer(4); seqSearch(arr, 0, 3, new Integer(3) );

ADSA: Generics/ General Collections A general-purpose collection class stores different kinds of elements. One way to implement the collection is to store the elements as Object instances. Problems: –A cast is required to access an element; –Accessing an element with the wrong cast will often cause a runtime error

ADSA: Generics/2 10 Object-based Store Example public class Store { private Object value; // data public Store () { value = null; } public Object getValue() { return value; } // set v as the new stored value public void setValue(Object v) { value = v; } public String toString() { return "value = " + value; } }

ADSA: Generics/2 11 Using the Store Store s1 = new Store(); s1.setValue("hello"); String msg1 = (String) s1.getValue(); Store s2 = new Store(); s2.setValue( new Time24() ); Time24 t2 = (Time24) s2.getValue(); String msg2 = s2.getValue(); //? Time24 t1 = (Time24) s1.getValue(); causes a runtime error

ADSA: Generics/ Better Collections with Generics In a generic collection, instances use a generic type T, which is fixed at compile- time to be a particular class. No cast is required when accessing a collection element –no possibility of runtime errors –any errors are caught at compile time, which means they can be fixed easily

ADSA: Generics/2 13 Generic Store Example public class Store { private T value; // data of type/class T public Store () { value = null; } // return the stored value as type T public T getValue() { return value; } // update the stored value public void setValue(T v) { value = v; } public String toString() { return "Value = " + value; } } All these T's must be the same class/type when a Store object is used.

ADSA: Generics/2 14 Using the Store Store ss = new Store (); ss.setValue("hello"); String msg = ss.getValue(); Store st = new Store (); st.setValue( new Time24() ); Time24 t = st.getValue(); String msg2 = st.getValue(); // warning Time24 t1 = ss.getValue(); causes a compile time error

ADSA: Generics/ Generic Interfaces Interfaces can use generic types: public interface Accumulator { public void add(T v); } Using the interface: public class Foo implements Accumulator

ADSA: Generics/2 16 AccumulatorTime Class public class AccumulatorTime implements Accumulator { private Time24 total; // time total public AccumulatorTime () { total = new Time24(); } // time is 0:00 public Time24 getTotal() { return total; } // add time v to total public void add(Time24 v) { total.addTime(v.getHour()*60 + v.getMinute()); } } store a Time24 total

ADSA: Generics/2 17 AccumulatorNumber Class public class AccumulatorNumber implements Accumulator { private double total; public AccumulatorNumber () { total = 0.0; } public double getTotal() { return total; } // add v to total as a double public void add(Number v) { total = total + v.doubleValue(); } } store a Number total

ADSA: Generics/2 18 Using the Accumulators import ds.time.Time24; public class UsingAccumlators { public static void main (String[] args) { Integer[] intArr = {7, 1, 9, 3, 8, 4}; String[] strArr = {"3:45", "2:30", "5:00"}; AccumulatorNumber accNumber = new AccumulatorNumber(); AccumulatorTime accTime = new AccumulatorTime(); :

ADSA: Generics/2 19 for (int i = 0; i < intArr.length; i++) accNumber.add(intArr[i]); System.out.println("Numeric total is " + accNumber.getTotal()); for (int i = 0; i < strArr.length; i++) accTime.add(Time24.parseTime(strArr[i])); System.out.println("Time total is " + accTime.getTotal()); } Numeric total is 32 Time total is 11:15

ADSA: Generics/ The Comparable Interface The Comparable interface is a standard way to compare objects. The interface defines a single method: public interface Comparable { int compareTo(T item); }

ADSA: Generics/2 21 compareTo() Meaning compareTo() should return an integer that is negative, 0, or positive. Meaning: –obj.compareTo(item) < 0 when obj < item –obj.compareTo(item) == 0 when obj == item –obj.compareTo(item) > 0 when obj > item

ADSA: Generics/2 22 Time24 Class with Comparable public class Time24 implements Comparable {... public int compareTo(Time24 item) //compares times { int time = hour*60 + minute; // use minutes int ttime = item.hour*60 + item.minute; // compare, returning -1, 0, or 1 if (time < ttime) return -1; else if (time == ttime) return 0; else return 1; }

ADSA: Generics/ Generic Methods A generic method includes a generic type modifier : public static T max(T a, T b) /* compare two objects of the same type/class, return the biggest */ {... } continued

ADSA: Generics/2 24 The modifier tells the compiler that T is a generic type, and the compiler checks that objects a and b and the return type are all the same. There's no need for if the method is inside a generic class, such as setValue() on slide 13.

ADSA: Generics/2 25 A Bad Generic max() javac prints an "unchecked cast" warning because this method is not type-safe –the code may cause a run-time error because a's class may not implement Comparable when object a is cast at run time public static T max(T a, T b) { if (((Comparable )a).compareTo(b) > 0) return a; else return b; } the compiler cannot be sure that the typing is correct

ADSA: Generics/2 26 // Rectangle does not implement // Comparable Rectangle rectA = new Rectangle(4,5); Rectangle rectB = new Rectangle(6,2); Rectangle r = max(rectA, rectB); // compiler time warning; // exception at runtime

ADSA: Generics/2 27 A Good Generic max() public static > T max(T a, T b) { if (a.compareTo(b) > 0) return a; else return b; } T is a bounded type The bound specifies that the generic type T must implement the Comparable interface.

ADSA: Generics/2 28 A Better Generic max() ? is a wildcard (unknown) public static > T max(T a, T b) { if (a.compareTo(b) > 0) return a; else return b; } "?" and "super" mean that T, or some superclass of T, must implement the Comparable interface.

ADSA: Generics/ Generic Searching/Sorting A generic sort method takes an array of type T. A generic search method also uses a target of type T. If the code uses compareTo() to compare objects, then the method must include the bounded wildcard modifier: >

ADSA: Generics/2 30 Generic Selection Sort public static > void selectionSort(T[] arr) { // index of smallest element in sublist int smallIndex; int idx; int n = arr.length; // idx has range 0 to n-2 for (idx = 0; idx < n-1; idx++) { // scan sublist starting at index idx smallIndex = idx; :

ADSA: Generics/2 31 // go through arr[idx+1] to arr[n-1] for (int j = idx+1; j < n; j++) /* if smaller element found, assign smallIndex to that position */ if (arr[j].compareTo(arr[smallIndex]) < 0) smallIndex = j; // swap smallest element into arr[idx] T temp = arr[idx]; arr[idx] = arr[smallIndex]; arr[smallIndex] = temp; }

ADSA: Generics/2 32 Generic binSearch () public static > int binSearch(T[] arr, int first, int last, T target) { int mid; // midpoint index T midvalue; // holds arr[mid] // save original value of last int origLast = last; // test for nonempty sublist while (first < last) { mid = (first+last)/2; midvalue = arr[mid]; :

ADSA: Generics/2 33 if (target.compareTo(midvalue) == 0) return mid; // found a match // find which sublist to search else if (target.compareTo(midvalue) < 0) // search lower sublist. set last last = mid; else // search upper sublist. set first first = mid+1; } return -1; // target not found }

ADSA: Generics/2 34 Generic Sort Examples import ds.util.Arrays; import ds.time.Time24; public class GenericSorting { public static void main (String[] args) { String[] strArr = {"red", "green", "blue"}; Integer[] intArr = {40, 70, 50, 30}; Time24[] timeArr = { new Time24(14,15), new Time24(10, 45), new Time24(22,00), new Time24(3,30)}; SalaryEmployee[] emp = { new SalaryEmployee("Dunn, Moira"," ",800), new SalaryEmployee("Garcia, Avey" " ",1200), new SalaryEmployee("Ye, Don", " ",2000)}; :

ADSA: Generics/2 35 Arrays.selectionSort(strArr); System.out.println("Sorted strings: " + Arrays.toString(strArr)); Arrays.selectionSort(intArr); System.out.println("Sorted integers: " + Arrays.toString(intArr)); Arrays.selectionSort(timeArr); System.out.println("Sorted times: " + Arrays.toString(timeArr)); Arrays.selectionSort(emp); for (int i=0; i < emp.length; i++) System.out.println(emp[i].payrollCheck()); }

ADSA: Generics/2 36 Output

ADSA: Generics/2 37 Employee and SalaryEmployee public class Employee implements Comparable { // vars are accessible by subclass methods protected String empName; : // compare social security numbers public int compareTo(Employee obj) { return empSSN.compareTo(obj.empSSN); } } continued

ADSA: Generics/2 38 public class SalaryEmployee extends Employee { // new vars private double salary; : // no compareTo() method here }