01-Basics

This note summarizes key points you need to know before writing classes.

Variables

Primitive Types

1. int: integers
2. double: decimals
3. boolean: true or false

Reference Types

1. String
2. Objects

The two types are different: Primitive Types store values, Reference Types store address (可以理解为快捷方式？

String 字符串

sequences of characters: letters, numbers, punctuation, spaces…

index: [0, length-1]

String name = "Eileen";

• numeric value can be concatenated to a string
• Whenever one of the arguments of the + operator is a string, the other one also becomes a string.
• 括号优先，从左往右算，没有遇到String就按正常的加法算

System.out.println(7 + 8 + (7 + 8) + "Hello" + 7 + 8 + (7 + 8))  //the result should be 30Hello7815

Escape Sequences 转义字符序列

Compare Strings

1. ASCII code: digit< capital letter< lowercase letter
2. String 的比较只能使用 .equals() 或者 compareTo()，不要使用 ==!

String Concatenations

• substring 方法的范围截止到 endIndex - 1
• 

Math Class

• Math.abs(-3.2) returns 3.2 //returns input type
• Math.pow(3, 2.0) returns 9.0 //returns a double regardless of input type; (base, exponent)
• Math.sqrt(9) returns 3.0 //returns a double
• Math.PI
  • a public variable of the type double approximately equal to $\pi$
• Math.random() returns a random number [0,1)
• 要产生一个[k, p]之间的随机整数: (int)(Math.random()*(p-k+1)+k)
• Probability:
  • General Rule: Math.random() < probability
  • e.g. 40% (2 methods)
    1. Math.random() < 0.4
    2. (int)(Math.random()*10)+1 ⇐ 4

Order of Operation

Boolean Expressions

• = is for assignment
• == is for equality testing

Logical Operators

• && (and 且)
• || (or 或)
• ! (not 非)
• 运算顺序
  1. arithmetic (+, -, x, /, %)
  2. relational (e.g. ==)
  3. logical (先算 && 再算 ||)
• 分配律
  • A || (B && C) = (A || B) && (A || C)

真值表

• Applying logical operators

  int age = 18;
  // if (12 <= age <= 18) // Error
  if (age >=12 && age <= 18) // Correct [12,18]

• Short-circuited evaluation ^8c5695
  • the second boolean value is not reached
  • e.g. false &&
  • e.g. true ||
  • can be used to avoid NullPointerException

	  String str = null;
	  if(str != null && str.length() > 10)
	  // if(str.length() > 10 && str != null) // you must check null first because if the string itself is null, evaluating "str.length()“ would cause a runtime error
	  {
	  System.out.println("something")
	  }

IMPORTANT

De Morgan’s Law 德摩根定理

• ! (A && B) is the same as !A || !B

• ! (A || B) is the same as !A && !B

Wrapper Class

• Autoboxing: automatic conversions between primitive values and corresponding wrapper objects

Integer obj3;
int num3 = 69;
obj3 = num3; //automatically creates an Integer object

• Unboxing: automatic conversions between wrapper objects and corresponding primitive values

Integer obj4 = new Integer(69);
int num4;
num4 = obj4; //automatically extracts the int value

Method

Method Overloading

several methods have the same method name with different parameter lists

• consider:

  1. number of parameters
  2. order of parameters
  3. type of parameters

• Methods that are exactly the same cannot exist at the same time. (when a method is called, Java decides which version of it to execute depending on the arguments given)

• return type 是 void 的 method

  • 不能用来给变量赋值，return type 和 data type 要对应 （特殊情况是 int 和 double：int 可以给 double 变量赋值，但是 double 不能给 int 变量赋值，因为 double 的范围比 int 大）
  • 不能被打印

Iteration 循环

While Loops

• executes statements repeatedly while the condition is true
• avoid infinite loops
• the return statement inside an iteration statement will halt the loop and exit the method or constructor

while (condition)
{
    //statements
}

For loops

• 可改写成 while loop, while loop 前需要 declaration

for (initialization; condition; update)
{
    //statements
}

• e.g.

for (int count = 1; count <= 5; count++)
{
	System.out.println (count);
 

Nested Loop

a loop that is declared inside another loop

• each time the outer loop runs once, the inner loop will go through a complete cycle
• 计算次数：
  • for (int i = 30; i < 300; i++)
  • i: [30, 300) or [30, 299]
  • 次数：300-30=270 or 299-30+1=270

If-statements

1. if () {}
2. if () {} else {}
3. if () {} else if {} else {}
4. if () {} if () {}

if statements 中，有 else 代表互斥，if 和 else 不能同时执行; 两个 if 代表并列

Common Algorithms

Reverse a String

public static void main(String[] args) {
	String str4 = "ap csa";
	String temp = "";
	for (int k = str4.length(); k > 0; k--) {
	
		temp = temp + str4.substring(k - 1, k);
	}
	System.out.println(temp);
}