171 Java Command Reference

Class Structure:

public class MyClassName
{
     // you can define a class variable here
     public static void main( )
     {
           // your statements will go here
           // your calls to method1 will have the form: method1()
     }
     public static void method1( )
     {
           // your statements will go here
     }
}

Create variables

• create using format: type name (a class can be a type)
• create arrays of a type using type[] arrayname = new type[number]
• primitive or class (boolean, double, char, integer, Strings)
• instance (outside methods) or local (parameter or inside method)
  • call instance variables with this. as prefix.
• variables can only have certain identifiers
  (no spaces in the name, can't start with a number, no spec characters, etc)
• ex: Scanner xa ; int y ; int x; String y;

Assign values to variables:

• = or use a method that updates
• identify array elements as arrayname[index]
  • get length with arrayname.length
• right side resolves to a value that goes into left side variable name
• ex: x = 3; xa = new Scanner(System.in)

Compare values:

• >,<,==,=>, =< for primitive types only
• equals or compareTo method for classes
• && means And ; || means Or. Can use parentheses to show precedence.
• ex: if (a < b && c < d || a < c) means: if either A is less than C OR ( A is less than B AND C is less than D ) it will pass the if statement

Decisions

• If ( ) {} else if ( ) {} else {}
• put decisions inside parentheses - resolve to boolean (true or false)
• ex: if (a < b) { x = 1; } else if ( c < d) { x = 2; } else {x = 3;}

Loops (Repeat)

• For ( starter ; keep going condition; incrementer){}
  • ex: for (int c = 1; c < 10; c++) { x = x + c;}
• While (keep going condition) {}
  • Be sure the keep going condition can become false somewhere inside the loop
  • Make sure the loop goes at least once
  • ex: while ( x < 10) { x = x - 1; a = a + 1;}
  • do / while will execute once all the time and then check the condition before restarting.
  • ex: do {x = x -1; a = a + 1; } while (x < 10);
• Sum construct:
  • zero out a variable before the loop (double sum = 0:)
  • add to that variable inside the loop (sum = sum + whateverYouWantToCount;)
  • print the total after the loop (System.out.println(sum);)
  • ex: x = 0; for (int c = 1; c < 10; c++) { x = x + c;} // and now x has the total of all c's

Make methods:

• public [static] returnType methodname (type var, type var){ }
• return needed if not returnType void
• static means the method cannot use instance variables (cuts off "this")
• input parameters are considered local variables. If they are arrays or objects, their address will be passed so the parameter will point to the caller's copy of the variable.
• constructors are special methods with no return type and methodname the same as the class name
• ex: public static int getNumber(){ return 3; }
• ex constructor of MyClass: public MyClass (int number) { this.number = number;}

Use methods

• variable = [class or object identifier].methodname(var, var);
• if void type, do not set equal to a variable
• input parameters must match contract
• input parameters should not be created as they are passed (meaning no (int x, String y), just (x,y))
• if the method is not static, you cannot call it with [class ], you must use [object identifier].
• ex: myScanner.nextLine(); anotherMethodinMyClass(6,2); new Scanner(System.in)

Make classes:

• name instance variables
• encapsulate with private (fence off anyone of another class from ever using these directly)
• create constructors: public methodname (type var, type var){ }
• use instance variables as this.variable name
• methods using "this" cannot be static

Use classes

• Scanner, Point, DecimalFormatter, Random, classes you create
• Create instances using constructor:
  • classname variable = new classname(var, var);
  • ex: Scanner myScanner = new Scanner(System.in);
• Call methods of an instance:
  • variable = object identifier.methodname(var, var);
  • ex: int x = myScanner.nextInt();

Print to the screen:

• System.out.println ( what you want to output ); // inside parentheses put "string" and variables separated with +
• System.out.print // does not go to the next line
• System.out.printf (what you want to output, variable names) (This wont be on the test)
• Can use DecimalFormatter to format inside println and print (Need to read this, not to create it)
• When printing an object, it will use the object's toString method to format the object's information
• ex: System.out.println("Hello, my name is " + myName + "My number is " + myNumber);

Comments:

• // -> comment line ex: // this is a comment
• /* xxx */ comment between marks

Read from screen:

• Import a scanner
• Create a scanner
• Print out a prompt to screen first
• Use scanner to read using next(), nextLine(), nextInt(), nextDouble()
• ex:

  import java.util.Scanner;
  Scanner s = new Scanner(System.in);
  System.out.println("enter a number");
  ex: int value1 = s.nextInt();

Syntax rules

• Every line ends with { or ;
• Only the following that we know end with {: if, else, for, while, method header, class header
• Inside parentheses, separate with comma only, except for the for statement, which separates with semi-colon
• Literals inside double quotes
• char inside single quotes

Tracing

• debug: step / step into and see values
• hand tracing: local variables and values inside a class
• follow code that creates objects and calls methods on the objects;
• -----> following them all the way into the class methods

Details on some classes we used:

Scanner: import java.util.*;
create instance: Scanner x = new Scanner(System.in);
call methods: int y = x.next();

Random: import java.util.Random;
create instance: Random x = new Random();
call methods: int y = x.nextInt(6);

Decimal Format: Library: import java.text.DecimalFormat; ---> (we did not use this much)
Create instance: DecimalFormat percentage2 = new DecimalFormat(“0.00%”);
Call methods: System.out.println(percentage2.format(.308)); // will print 30.80

0 = required position
# = show it if you have it
% = make it a % (mult by 100)
E = E notation (scientific)

• stringvar.charAt(3) // will be a character
• stringvar.length() // will be an integer
• stringvar.equals(somethingOtherString) // will be either true or false (a boolean)
• stringvar.toUpperCase() // will be a String that is all uppercase
• stringvar.toLowerCase() // will be a String that is all lowercase
• stringvar.subString(2) // will be a String starting in position 2 to the end
• stringvar.subString(2, 5) // will be a String starting in position 2 and ending in position 4
• stringvar.indexOf("xx",2) // return the index of the first place xx is found, starting the search in position #2.
• Format
  • ex: newString = myOriginalString.toLowerCase();

Math:

• absolute value ex: myDoubleVar= Math.abs(#);
• random ex: myDoubleVar = Math.random();
• minimum: Math.min(#,#)
• maximum: Math.max(#,#)
• raise to a power: Math.pow(#,#) : Note: The first number is raised to the second number
  
• knows PI and E

Graphics:

Picture It

•import picture knowledge: javalib.worldimages.*;
•import color knowledge: java.awt.Color;
•Make WorldImages using AImage.make<shape> , AImage.makeFromFile
•Place images on top of each other using <your world image>. place (<your other world image> , x,y)
•x indicates how far to the right; y indicates how far down (positive)
•Show your image with <your world image>.show()