# Java Basic

## Null 和 0 的差異

`List a == null`是指List位址是空，而`List a == 0` 是List 位址為0xFDJJLJDKLASJDLA，空間已經開出來了裡面卻沒放東西

## Prefix-increment and post increment 前置遞增和後置遞增的差異

```java
public class UnaryOperatorDemo {
    public static void main(String[] args) {
        int age = 10;
        int var = age++;
        System.out.println(age);
        System.out.println(var);
    }
}
```

`age++`是後置算法，等同於

```java
var = age;
age = age + 1;
```

`age`先指定給`value`後才遞增，所以輸出是11, 10

[Reference](http://www.codedata.com.tw/book/java-basic/index.php?p=ch5-1)

## overloading & overriding

* overloading: 藉由接收的參數串列之型態或數量之不同，以求達到多個函式可共用相同函式名稱 (回傳值可不同)。目的為降低所需命名的函式名稱

```java
void print(int i) {…}
void print(double d) {…}
void print(char c) {...}
```

* overriding: 子類別將父類別函式重新定義以符合自身所需(回傳值必須相同)。目的為利用父類別來動態操作子類別的函式，以達多型的效果。

```java
class Person {
  public void iam()

  {System.out.println(“I am a person.”);}

}

class Superman extends Person {

  public void iam()

  {System.out.println(“I am a Superman.”);}

}
```

## 記憶體

1. Java中**-128**到**127**之間在裝箱後會存在記憶體中被重複使用。以外的值則是程式在運行時才會建立一個新的物件
2. 比較兩個物件大小必須使用`equals()`，使用`==`則是比較兩個物件是否比較到同一個參考名稱

[Reference](http://www.codedata.com.tw/book/java-basic/index.php?p=ch8-2)

## Class使用方式比較

![表格](https://www.javaworld.com.tw/jute/upload/2009/01/21/21849063.gif)

## Random

```java
Random ran = new Random();
int r = ran.nextInt(n) // create number 0~(n-1)
```

## String 經常變動使用 StringBuilder

```java
public static String intToString(int x){
  boolean isNegtive = false;
  if (x<0)
    isNegtive = true;
    x = -x;
  }

  StringBuilder s = new StringBuilder();
  while(x != 0){
    s.append((char)('0'+ x%10))}
    x /= 10;

    if (Negtive){
      s.append('-');
  }

  s.reverse();
  return s.toString();
}
```

## Comparator and Comparable

1. Comparable: 類別本身繼承comparable，改寫compareTo，實現在類別內部

```java
public class Student implements Comparable {
     String name;
     int age
     public int compareTo(Student another) {
          int i = 0;
          i = name.compareTo(another.name); 
          if(i == 0) { 
               return age - another.age;
          } else {
               return i; 
          }
     }
}

// 可以使用Collections.sort(StudentList)排序
```

1. Comparator: 指定comparator告知類別如何排序，改寫compare，實現在類別外部

```java
public class Student{
     String name;
     int age
}
class StudentComparator implements Comparator { 
     public int compare(Student one, Student another) {
          int i = 0;
          i = one.name.compareTo(another.name); 
          if(i == 0) { 
               return one.age - another.age;
          } else {
               return i;          }
     }
}
//Collections.sort(StudentList , new StudentComparator())
//指定comparator
```

* 使用binarySearch必須指定Comparator &#x20;

  `Collection.binarySearch(Object, target, CompGPA)`

```java
private static Comparator<student> compGPA = new Comparator<student>(){
  public int compare(student s1, student s2){
};
```


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