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

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); }} ///:~

9

47

n1

n2

9

47

n1

n2

1: n1.i: 9, n2.i: 47 2: n1.i: 47, n2.i: 47 3: n1.i: 27, n2.i: 27

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

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

• 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:Equivalence.javaimport 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); }} ///:~

//: c03:EqualsMethod.javaimport 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));

}} ///:~

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 shif

t >>>, which uses zero extension

1 0 1 0 1 0 1 0

>> 1 1 0 1 0 1 0 1

>>> 0 0 1 0 1 0 1 0 1

• 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

• Exercise６． Write a function that takes two String ar

guments, 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.