02-Basic Java.md

Reading 2: Basic Java

Software in 6.031

Safe from bugs	Easy to understand	Ready for change
Correct today and correct in the unknown future.	Communicating clearly with future programmers, including future you.	Designed to accommodate change without rewriting.

Snapshot diagrams

primitive values

object values

object 用 圈表示， primitive 用字母表示， 值用 箭头表示

mutating values vs. reassigning variables

重新赋值 vs 更改元素

== vs. equals()

== 对于 object 来说，对比的是其引用地址

.equals 对比的是其值。

Java Collections

list

Java	description	Python
int count = lst.size();	count the number of elements	count = len(lst)
lst.add(e);	append an element to the end	lst.append(e)
if (lst.isEmpty()) ...	test if the list is empty	if not lst: ...

Map

Java	description	Python
map.put(key, val)	add the mapping key → val	map[key] = val
map.get(key)	get the value for a key	map[key]
map.containsKey(key)	test whether the map has a key	key in map
map.remove(key)	delete a mapping	del map[key]

Set

Java	description	Python
s1.contains(e)	test if the set contains an element	e in s1
s1.containsAll(s2)	test whether s1 ⊇ s2	s1.issuperset(s2) s1 >= s2
s1.removeAll(s2)	remove s2 from s1	s1.difference_update(s2) s1 -= s2

literals

python 可以直接通过 【】 创建列表

java的 String[] arr = { "a", "b", "c" }; 创建的是array，

而如果想要创建 list、set、map的话，需要使用

List.of("a", "b", "c");
Set.of("a", "b", "c");
Map.of("apple", 5, "banana", 7);

泛型创建 generics

List<String> cities;
Set<Integer> numbers;
Map<Sring, Turtle> turtles;

ArrayLists and LinkedLists: creating Lists

List, Set, Map 是 interface， 不包含具体实现，而 ArrayList 和 LinkedList 则是具体实现。

HashSets and HashMaps: creating Sets and Maps

Java API documentation

Overview (Java SE 12 & JDK 12 ) (oracle.com)