【CS 61b study notes 2】Linked List & Array

Basic data info

Declaring a Variable

• Variable Definition

  • className variableName = new className ();
    • for reference type
    • e.g. : IntList L1 = new IntList(5, null);
  • primitiveType variableName = value;
    • primitive types are built-in data type in java and can be used directly without using any “new” keyword.
    • e.g. : int x = 10;
  • Some rules
    • A variable name can consist of Capital letters A-Z , lowercase letters a-z , digits 0-9 and two special character such as _underscore and $ dollar sign.
    • First character can not be digit
    • Can NOT use Blank spaces and java keywords
    • No limit in the length ,but by convention 4 to 15 chars
    • Always exist on the left-hand side of assignment operators =
    • Some vaild example : variableName , variablename , _variableName ,$variableName , variableName1, variableName_1, VARIABLENAME

• Example

  String str1 = "abc";
  String str2 = "abc";
  
  String str3 = new String("abc");
  String str4 = new String("abc");
  String str5 = str3;
  
  System.out.println(str1 == str2); // true
  System.out.println(str3 == str4); // false
  System.out.println(str1 == str3); // false
  System.out.println(str3 == str5); // true
  System.out.println(str3.equals(str4)); // true
  System.out.println(str3.equals(str1)); // true
  System.out.println(str3.equals(str5)); // true
  System.out.println(str1.equals(str2)); // true
  
  

• For the reference type ,“==” checks the address (or the object), while .equal() checks the value(or the content).

• When we assign “abc” to str1 , java will not build a new string object immediately .Instead, java checks whether there exists “abc” in the stack constant field first. If thers is no “abc” , it will build a constant “abc” in the stack.

• The assignment of str2 is after the str1 , so ‘abc’ has already existed in the constant field. str2 will point at the

• the new keyword creates a new String object and returns the address of the overall object for assignment to str3 . Although the values of str1 ,str3 and str4 are both “abc” , the addresses are different.

• Every time we use new keyword, java will create a completely different object and assign a new address to it , which means the address that java assigns to the new object is independent of what the object contains.

When we declare a variable of a certain type, java finds a contiguous block with exactly enought bits to hold a thing of that type. Each data type in java holds a different number of bits. Unlike C or python ,java can not access the exact address of the data . In java , the exact memory address is below the level of the abstraction accessible to us.

• Local Variable
  • Local Variables can be declared in methods, code blocks, constructors, etc in java.
  • Local variables do not have any default values in java
  • Local variable can be used only after it has been assigned
  • declaring a variable without initialization : int num
  • Initializing the variable : int num = 100;

// error : variable might not have been initialized
public class test{
   
	public static void main(String[] args){
   
		// local variable must be initialized before using ,otherwise complier errors
		int a ;
		System.out.println(a);
	}
}

• Declare a static global primitive type variable without initialization

// output : 0
public class test{
   
	// static declaration for global int variable is important owing to  the static main function
	static int a ;
	public static void main(String[] args){
   
		System.out.println(a);