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Objects. contains data and methods Class – type of object Create class first Then object or instance String – defined class String s; // creates instance of string s.length () calls method defined in class Most methods in String return a new string. Examples of objects.
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Objects • contains data and methods • Class – type of object • Create class first • Then object or instance • String – defined class • String s; // creates instance of string • s.length() calls method defined in class • Most methods in String return a new string
Examples of objects • Scanner console = new Scanner (System.in); • File f = new File(“hamlet.txt”); • Scanner input = new Scanner (f); • Scanner input = new Scanner (new File(“hamlet.txt”)); • String s1 = new String (“hello”); • Or String s1 = “hello”;
> import java.awt.Point; // auto-import > Point p = new Point(3,8); > System.out.println(p); java.awt.Point[x=3,y=8] > p.translate(-1,-2); > System.out.println(p); java.awt.Point[x=2,y=6]
Reference semantics • int x = 3; //primitive type • x 3 //value semantics • Point p = new Point(3,8); //object • p x 3 y 8 //reference semantcis • P refers to the object • When working with objects, always working with reference to data rather than data itself
import java.awt.Point; public class PointExamp1 { public static void main(String[] args) { Point p1 = new Point(3,8); Point p2 = new Point(); Point p3 = p2; System.out.println("p1 = " + p1); System.out.println("p2 = " + p2); System.out.println("p3 = " + p3); p1.x = -1; p1.y = 1; p2.x = -2; p2.y = 2; p3.x = -3; p3.y = 3; System.out.println("p1 = " + p1); System.out.println("p2 = " + p2); System.out.println("p3 = " + p3); } > java PointExamp1 p1 = java.awt.Point[x=3,y=8] p2 = java.awt.Point[x=0,y=0] p3 = java.awt.Point[x=0,y=0] p1 = java.awt.Point[x=-1,y=1] p2 = java.awt.Point[x=-3,y=3] p3 = java.awt.Point[x=-3,y=3]
Multiple objects • Point p1 = new Point(3,8); // separate instance • Point p2 = new Point(); //separate instance • Point p3 = p2; // points to p2
Objects as parameters to Methods public static void manipulate (Point p) { p.x = 99; p.y = 99; } Point p1 = new Point(3,3); manipulate(p1);
import java.awt.Point; public class PointExamp1 { public static void main(String[] args) { Point p1 = new Point(3,8); Point p2 = new Point(5,2); silly(p1); silly(p2); Point p3 = p1; silly(p3); System.out.println("p1 = " + p1); System.out.println("p2 = " + p2); System.out.println("p3 = " + p3); } public static void silly (Point p) { int temp = p.x; p.x = p.y; p.y = temp; } }
Evaluate the following • What are the x and y coordinates of the Points p1,p2, and p3 after the following? Point p1 = new Point(17,9); Point p2 = new Point(4,-1); Point p3 = p2; p1.translate(3,1); p2x = 50; p3.translate(-4,5)
public class StringComparision1 { public static void main(String[] args) { String name1 = "Bob"; String name2 = new String("Bob"); String name3 = "Bob"; // 1st case if (name1 == name2) { System.out.println("The strings are equal."); } else { System.out.println("The strings are unequal."); } // 2nd case if (name1 == name3) { System.out.println("The strings are equal."); } else { System.out.println("The strings are unequal."); } } }
public class Temperature { public static void main(String[] args) { double tempf = 98.6; double tempc= 0.0; ftoc(tempf,tempc); System.out.println(“Body temp in C is: “ + tempc); } public static void ftoc(double tempf, double tempc) { tempc = (tempf – 32) * 5/ 9; } }
Evaluate the following • Math.abs(-1.6) • Math.pow(6,2) • Math.max(7,4) • Math.min(-2,-5) • Math.sqrt(64) • Math.abs(2 + -4) • Math.sqrt(16) * Math.max(Math.abs(-5),Math.abs(-3))
