Time is so fast and now it is the week 5, and the mid-term exam for the Mini 1 semester is coming. I have used a couple of hours to look though the slides before visiting the Kangaroo island. And the day before Java exam, I practiced hundreds of sample questions. Now I put summary here. I have encountered two problems about scanner.

• get stuck in a loop if an exception is caught

        Scanner sc = new Scanner(System.in);
        while (true) {
            System.out.println("I will stuck in the loop");
            try {
                sc.nextInt();
            } catch (InputMismatchException e) {
                continue;
            }
            System.out.println("I will no be diplayed");
        }
  

  Reason: This is becasue nextInt() does not comsume the invalid input. It is right there and will be received by the nextInt() the next iteration. So the exception will be caught each loop, and the loop always continue.

  How to solve: You can use a next() or nextLine() to clear the input stream.

+

I wanted to read the java source code in Eclipse, but failed to install Apple java package in my Mac. Finally I downloaded the linux JDK package at Oracle’s website in which there is a src.zip file that contains source code.

Use Array.copyOf() to copy array.

	public String(char value[]) {
        this.value = Arrays.copyOf(value, value.length);
    }

• subsequent modification of the character array does not affect the newly created string. ***

First specify method(int arg1,int arg2)

then call it in method() { method(1, 0) }

rather than conversing this order.

Calculate square root.

float square = 4;    // number to find sq root of
   float squareRoot = square;    // first guess
   while (squareRoot * squareRoot - square > 0.001) { // How accurate?
       squareRoot = (squareRoot + square/squareRoot)/2;
       System.out.println("Next try will be " + squareRoot);
   }

Use static methods and variables without writing class name and dot notation before them.

eg. Controller.turnLeft –> turnLeft

	static import packagename.Controller.*;

Object o1 = null;
Object o2 = null;
System.out.println(o1==o2);//true
o1.equals(o2);//false

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

A good practice is to design and write methods that take the most generic form of your object as possible.

x =+ 1; x= + 1 are legal ***

1. Which of these statements are incorrect? a) Assignment operators are more efficiently implemented by Java run-time system than their equivalent long forms. b) Assignment operators run faster than their equivalent long forms. c) Assignment operators can be used only with numeric and character data type. d) None View Answer

Answer: d *** & > ^ > ?:

() > ++ > * > » ** two case constants in the same switch can’t have identical values. **

byte, char, and short values are promoted to int before the operation.

	short a, b, c;
	a = 1 ;
	b = 2 ;
	c = a + b ; //compiler error

All literal floating point values are viewed as double.

float float1 = 27.9;	//compiler error

Variable in a switch can be only of type char, byte, short, int, or String. ** Constructors does not have any return type, not even void. ** A class member declared protected becomes member of subclass of private type. ***

instanceof must be followed with a type instead of an object.

covariance

contravoriance

invariance

Why does it.next() throw java.util.ConcurrentModificationException?