3: Controlling Program Flow

3: Controlling Program Flow • Using Java operators • Mathematical operators • Relational operators • Logical operators • Primitive type: ALL (the same with C) • String: + += • All Objects: = == != • Assignment/Aliasing during method calls • Primitive type: byvalue (copy) • Objects: byRef • Sample-1 sample-2

9 n1 47 n2 9 n1 47 n2 class Number { int i; } public class Assignment { public static void main(String[] args) { Number n1 = new Number(); Number n2 = new Number(); n1.i = 9; n2.i = 47; System.out.println("1: n1.i: " + n1.i + ", n2.i: " + n2.i); n1 = n2; System.out.println("2: n1.i: " + n1.i + ", n2.i: " + n2.i); n1.i = 27; System.out.println("3: n1.i: " + n1.i + ", n2.i: " + n2.i); } } ///:~ • 1: n1.i: 9, n2.i: 47 • 2: n1.i: 47, n2.i: 47 • 3: n1.i: 27, n2.i: 27

public class PassPrimitive { static void f(char y) { y = 'z'; } public static void main(String[] args) { char x ; x = 'a'; System.out.println("1: x: " + x); f(x); System.out.println("2: x: " + x); } } ///:~ 1: x: a 2: x: a class Letter { char c; } public class PassObject { static void f(Letter y) { y.c = 'z'; } public static void main(String[] args) { Letter x = new Letter(); x.c = 'a'; System.out.println("1: x.c: " + x.c); f(x); System.out.println("2: x.c: " + x.c); } } ///:~ 1: x.c: a 2: x.c: z

Relational operators • All type （including boolean）: == != • All primitive type but boolean: All Relational operators • < > <= >= == != • Logical Operators • AND : && • OR : || • NOT : !

Testing object equivalence • Primitive type: compare value(== !=) • int i=2; if ( i= =2) … • Objects: compare reference(handle) == != • String s=“AAA”; String ss=s; if(s = = ss)… • String s=“AAA”; if(s = = new String(“AAA”) )… • SPECIAL CASE: String s=“AAA”; if(s = = “AAA”)… • Objects: compare value xxx.equals • String s=“AAA”; if(s .equals( “AAA”) )… • (For user-defined class,) method equals must be defined beforehand • If haven’t redefined, default behavior of base class: compare byref

//: c03:EqualsMethod.java import com.bruceeckel.simpletest.*; public class EqualsMethod { public static void main(String[] args) { Integer n1 = new Integer(47); Integer n2 = new Integer(47); System.out.println(n1.equals(n2)); } } ///:~ //: c03:Equivalence.java import com.bruceeckel.simpletest.*; public class Equivalence { public static void main(String[] args) { Integer n1 = new Integer(47); Integer n2 = new Integer(47); System.out.println(n1 == n2); } } ///:~

class Number { int i; } public class EqualsMethod { public static void main(String[] args) { Number n1 = new Number (); n1.i=47; Number n2 = new Number (); n2.i=47; System.out.println( n1 ==n2 ); System.out.println(n1.equals(n2)); } } ///:~

Short-circuiting • The result of a logical expression may be determined without evaluation of the latter operands. • if(test1(0) && test2(2) && test3(2))

import com.bruceeckel.simpletest.*; public class ShortCircuit { static boolean test1(int val) { System.out.println("test1(" + val + ")"); System.out.println("result: " + (val < 1)); return val < 1; } static boolean test2(int val) { System.out.println("test2(" + val + ")"); System.out.println("result: " + (val < 2)); return val < 2; } static boolean test3(int val) { System.out.println("test3(" + val + ")"); System.out.println("result: " + (val < 3)); return val < 3; } public static void main(String[] args) { if(test1(0) && test2(2) && test3(2)) System.out.println("expression is true"); else System.out.println("expression is false"); } } ///:~ test1(0) result: true test2(2) result: false expression is false

Bitwise operators • All primitive are signed, no unsigned type • Java has also added the unsigned right shift >>>, whichuses zero extension

String operator + • There’s one special usage of an operator in Java: the + operator can be used to concatenate strings String sString = "x, y, z "; System.out.println(sString + “abc”); • If an expression begins with a String, then all operands that follow must be Strings • If needed, the compiler will turn a quoted sequence of characters into a String

int x = 0, y = 1, z = 2; String sString = "x, y, z "; //String sString = new String("x, y, z “); System.out.println(sString + x + y + z); Here, the Java compiler will convert x, y, and z into their String representations instead of adding them together first. And if you say: System.out.println(x + sString); Java will turn x into a String. Q1:System.out.println(x + y + z+sString); What will be outputed? class Letter { char c; } Q2:Letter object = new Letter(); System.out.println(sString+object); What will be outputed? sString+object.toString() x, y, z Letter@xxxxx

Casting operators • Java allow you to cast any primitive type to any other primitive type except for boolean. while(x = y) { // .... } int i=10; long L=(long) i; int k=(int)L • Java has no “sizeof” • Reason: all the data types are the same size on all machines. • Execution control • break and continue (label) • The infamous “goto” is not supported

Exercise ６．Write a function that takes two String arguments, and uses all the boolean comparisons to compare the two Strings and print the results. For the == and !=, also perform the equals() test. In main(), call your function with some different String objects.

3: Controlling Program Flow

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