Date: March 20, 2023

Topic: Practice 1

Recall

Notes

• 在function{}里面的object叫做local variable
• 在class 下的叫做member variable = filed
• call stack：函数的互相调用，local variable：primitive type，object的reference
• heap：object的值，field都存heap；
• class存在method area
• 常量使用UPPER_SNAKE_CASE
• 其他时候变量都是lowerCamelCase
• constructor是一个特殊的method，它必须要与class同名，而且没有return，constructor可以省略，意味着所有filed都有一个初始值，零假空。

<aside> 📌 SUMMARY:

</aside>

Date: November 5, 2019

Topic: Practice

Recall

Static

array VS Arrays VS Array

Exceptions

Notes

• Members(fields, methods, classes) belong to class, not object(instance).

• Arrays.sort(array): use binary search, TC: O(nlogn)

• new array: int[] = new int[3]

• array length cannot be changed after the array object is created. (final).

• Arrays: Java里放Array function的class

• Array: Java里的class

• array是一个object

• ArrayIndexOutOfBoundException!

• NullPointerException!

<aside> 📌 SUMMARY:

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Date: March 29, 2023

Topic: ArrayList & Interface

Recall

ArrayList

Abstract class and interface

Notes

• ArrayList is regarded as a resizable array.

  • maintain an array with potential unused cells
  • will expand the array if there is no unused cells available
    • by replacing the maintained array with a new larger array(1.5 times larger by default)

• class ArrayList<E> E: generic type (any reference type), but not primitive type.

• 如果不用generic type，默认存储和读取的是Object type

• 注意区分

  • size：目前装进去的元素个数
  • current capacity：能装元素个数的目前的上限

• get(int index): array[index] TC: O(1)

• set(int index, E e): array[index] = e TC: O(1)

• add(E e) - amortized O(1), 如果需要扩容，时间复杂度是O(n)，但是均摊下来也是O(1)

• add(int index, E e) index → [0, size] —worst O(n)

• remove(int index) index → [0, size] —worst O(n)

• when use ArrayList or LinkedList

  • if you have a lot of random access operation, use ArrayList
  • if you always add/ remove at the end, use ArrayList
    • 即使时间复杂度相同，用ArrayList
  • 尽量不要用Stack和Vector，如果要用Vector，改成用ArrayList，把用Stack改成用Deque

• implements 实现

• @Override，重写

• 一个class可以implements多个interface，但是只能inheritance一个super class，把interface理解成技能包

• Benefits: More flexible, more extensible

• 不能用interface去 instantiate一个object，但是class可以

<aside> 📌 SUMMARY: 优先级：int[]→ArrayList→LinkedList, 尽量不要使用LinkedList，除非你一定要用.next，因为LinkedList，overhead大，locality 差

</aside>

Date: April 24, 2023

Topic: Inheritance, Polymorphism, Access Modifier, Exceptions

Recall

Inheritance

Override

Polymorphism多态

Overload

Abstract class vs. interface

<aside> 📌 SUMMARY:

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