CS112-2012S-03 More Fun With Classes 103-0: Java Programs• Java programs are a collection of classes• Each class is a Template, not an Object• Can’t use a class until we create an instance (call “new”)• Each class contains methods and data03-1: Java Control Structures• If:if (<test>)<statement>orif (<test>)<statement1>else<statement2>• A statement is either a single statment (terminated with ;), or a block of statements inside { }03-2: If test• What can we have as the test of an if statement?• Boolean valued expression03-3: If test• What can we have as the test of an if statement?• boolean variable• comparison x < y or x != 0• Combination of boolean expressions, using not (!), and (&&), or (||)03-4: Boolean Variables• Hold the value true or false• Can be used in test (if, while, etc)boolean b;boolean c;b = true;c = b || false;b = (3 < 10) && (5 > 11);c = !b && c;03-5: if Gotchas• What is (likely) wrong with the following code?CS112-2012S-03 More Fun With Classes 2if (x != 0)z = a / x;y = b / x;03-6: if Gotchas• What is (likely) wrong with the following code?if (x != 0){z = a / x;y = b / x;}• Moral: Always use {} in if statements, even if they are not necessary03-7: while loopswhile(test){<loop body>}• Evaluate the test• If the test is true, execute the body of the loop• Repeat• Loop body may be executed 0 times03-8: do-while loopsdo{<loop body>} while (<test>);• Execute the body of the loop• If the test is true, repeat• Loop body is always executed at least once03-9: while vs. do-while• What would happen if:• Found a while loop in a piece of code• Changed to to a do-while (leaving body of loop and test the same)• How would the execution be different?03-10: while vs. do-while• What would happen if:CS112-2012S-03 More Fun With Classes 3• Found a while loop in a piece of code• Changed to to a do-while (leaving body of loop and test the same)• How would the execution be different?• If the while loop were to execute 0 times, do-while will execute (at least!) one time• If the while loop were to execute 1 or more times, should to the same thing ...• ... except if the test had side effects (stay tuned for more on this in coming weeks)03-11: for loopsfor (<init>; <test>; <inc>){<body>}• Equivalent to:<init>while(<test>){<body><inc>}03-12: for loopsfor (number = 1; number < 10; number++){System.out.print("Number is " + number);}• Equivalent to:number = 1;while(number < 10){System.out.print("Number is " + number);number++;}03-13: Calculator Example• Create a calculator class that has methods that allow you to:• add 2 numbers• multiply 2 numbers (without using the * operator!)• calculate xn(power function)03-14: Calculator Example II• Add to previous calculator example:• Two instance varaibles, firstOperand and secondOperandCS112-2012S-03 More Fun With Classes 4• New versions of add, multiply, power that take as inputs the instance variables, and return appropriatevalues03-15: Overloading Methods• You can have > 1 method with the same name• As long as the rest of the method signature, number and types of parameters, are different• Constructors can also be overloaded03-16: Overloading Constructorspublic class Calculator{int firstOperand;int secondOperand;public Calculator(){this.firstOperand = 0;this.secondOperand = 0;}public Calculator(int first, int second){this.firstOperand = first;this.secondOperand = second;}}03-17: Overloading Methodspublic int power(){return power(this.firstOperand, this.secondOperand);}public int power(int x, int y){int result;for (result = 1; y > 0; y--){result = multiply(result, x);}return result;}03-18: Overloading Methods• Note that the version of power without parameters called the version of power with parameters• Why is that a good idea?03-19: Overloading Methods• Note that the version of power without parameters called the version of power with parameters• Why is that a good idea?• Both versions of power do the same thing• If you change one, don’t need to change the other• Big problem in industrial code – more than one code path that does the same thing, fix a bug in one,might not fix the same bug in the other03-20: Random Java Goodness• File name needs to be <ClassName>.java• Class Calculator needs to be in file Calculator.javaCS112-2012S-03 More Fun With Classes 5• No spaces!03-21: Random Java Goodness• Static methods are very different from non-static methods• Can be somewhat confusing to have both static and non-static methods in the same class• We encourage a “Driver” class which contains a single static method main• Could place the static main in one of the other classes in the project – code would compile and run justfine, though it is a little
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