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还有一份JAVA核心知识点整理(PDF):JVM,JAVA集合,JAVA多线程并发,JAVA基础,Spring原理,微服务,Netty与RPC,网络,日志,Zookeeper,Kafka,RabbitMQ,Hbase,MongoDB,Cassandra,设计模式,负载均衡,数据库,一致性哈希,JAVA算法,数据结构,加密算法,分布式缓存,Hadoop,Spark,Storm,YARN,机器学习,云计算…
| byte | Byte |
| short | Short |
| int | Integer |
| long | Long |
| float | Float |
| double | Double |
| char | Character |
| boolean | Boolean |
2.2 包装类的使用,装箱(boxing)和拆箱(unboxing)
装箱 装包 : 把简单类型 ========> 包装类类型
拆箱 拆包 : 把包装类类型 =========> 简单数据类类型的数据
代码:
public static void main(String[] args) {
Integer a = 123;//装箱 装包[隐式的]
int b = a;//拆箱 拆包[隐式的]
System.out.println(“====================”);
//装箱 装包[显式的]
Integer a2 = Integer.valueOf(123);
Integer a3 = new Integer(123)
//拆箱 拆包[显式的]
int b2 = a2.intValue();
double d = a3.doubleValue();
}
==================================================================================
【说明】
**1.ArrayList实现了RandomAccess接口,表明ArrayList支持随机访问2.ArrayList实现了Cloneable接口,表明ArrayList是可以clone的
3.ArrayList实现了Serializable接口,表明ArrayList是支持序列化的
4.和Vector不同,ArrayList不是线程安全的,在单线程下可以使用,在多线程中可以选择Vector或者CopyOnWriteArrayList
5.ArrayList底层是一段连续的空间,并且可以动态扩容,是一个动态类型的顺序表**
==================================================================================
| 方法 | 解释 |
| :-- | :-- |
| ArrayList() | 无参构造 |
| ArrayList(Collection<? extends E> c) | 利用其他 Collection 构建 ArrayList |
| ArrayList(int initialCapacity) | 指定顺序表初始容量 |
有三种遍历方式:for循环,foreach,迭代器
public static void main(String[] args) {
ArrayList arrayList = new ArrayList<>();
arrayList.add(“hello”);
arrayList.add(“+”);
arrayList.add(“world!”);
System.out.println(arrayList);
System.out.println(“1.for循环”);
for (int i = 0; i < arrayList.size(); i++) {
System.out.print(arrayList.get(i)+" ");
}
System.out.println();
System.out.println(“2.foreach”);
for (String s:arrayList) {
System.out.print(s+" ");
}
System.out.println();
System.out.println(“3.使用迭代器的方法”);
Iterator it = arrayList.iterator();
while(it.hasNext()){
System.out.print(it.next()+" ");
}
System.out.println();
System.out.println(“4.迭代器List相关打印”);
ListIterator it2 = arrayList.listIterator();
while (it2.hasNext()){
System.out.print(it2.next()+" ");
}
| 方法 | 解释 |
| :-- | :-- |
| boolean add(E e) | 尾插 e |
| void add(int index, E element) | 将 e 插入到 index 位置 |
| boolean addAll(Collection<? extends E> c) | 尾插 c 中的元素 |
| E remove(int index) | 删除 index 位置元素 |
| boolean remove(Object o) | 删除遇到的第一个 o |
| E get(int index) | 获取下标 index 位置元素 |
| E set(int index, E element) | 将下标 index 位置元素设置为 element |
| void clear() | 清空 |
| boolean contains(Object o) | 判断 o 是否在线性表中 |
| int indexOf(Object o) | 返回第一个 o 所在下标 |
| int lastIndexOf(Object o) | 返回最后一个 o 的下标 |
| List subList(int fromIndex, int toIndex) | 截取部分 list |
public static void main(String[] args) {
ArrayList arrayList = new ArrayList<>();
System.out.println(“==尾插e=”);
arrayList.add(“a”);
arrayList.add(“b”);
arrayList.add(“c”);
System.out.println(arrayList);
System.out.println(“==将 e 插入到 index 位置=”);
arrayList.add(0,“qwe”);
System.out.println(arrayList);
System.out.println(“==尾插 c 中的元素=”);
ArrayList arrayList1 = new ArrayList<>();
arrayList1.add(“!!!”);
arrayList.addAll(arrayList1);
System.out.println(arrayList);
System.out.println(“==删除 index 位置元素=”);
System.out.println(arrayList.remove(0));
System.out.println(arrayList);
System.out.println(“==删除遇到的第一个 o=”);
System.out.println(arrayList.remove(“a”));
System.out.println(arrayList);
System.out.println(“==获取下标 index 位置元素=”);
System.out.println(arrayList.get(1));
System.out.println(“==将下标 index 位置元素设置为 element=”);
System.out.println(“原来要修改的数据:”+arrayList.set(1,“d”));
System.out.println(arrayList);
System.out.println(“==判断 o 是否在线性表中=”);
System.out.println(arrayList.contains(“!!!”));
System.out.println(“==返回第一个 o 所在下标=”);
System.out.println(arrayList.indexOf(“!!!”));
System.out.println(“==返回最后一个 o 的下标=”);
System.out.println(arrayList.lastIndexOf(“!!!”));
System.out.println(“==截取部分 list=”);
List sub = arrayList.subList(0,2);//左闭右开
System.out.println(sub);
System.out.println(“==情况=”);
arrayList.clear();
System.out.println(arrayList);
}
运行结果:
**结论:
如果ArrayList调用无参的构造方法new ArrayList() , 那么顺序表的大小是0.当第一次add的时候, 整个顺序表才变为了10;
当这10个放满之后,才开始扩容,以1.5倍的方式扩容.
如果调用的是给定容量的构造方法new ArrayList(13) , 那么顺序表的大小就是给定容量的大小,如果放满了,还是以1.5倍进行扩容.**
=====================================================================================
import java.util.ArrayList;
class Student{
private String name;
private String classes;
private double score;
public Student(String name, String classes, double score) {
this.name = name;
this.classes = classes;
this.score = score;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getClasses() {
return classes;
}
public void setClasses(String classes) {
this.classes = classes;
}
public double getScore() {
return score;
}
public void setScore(double score) {
this.score = score;
}
@Override
public String toString() {
return “Student{” +
“name='” + name + ‘’’ +
“, classes='” + classes + ‘’’ +
“, score=” + score +
‘}’;
}
}
public class TestDemo10 {
public static void main1(String[] args) {
ArrayList students = new ArrayList<>();
students.add(new Student(“niubi”,“102-1”,98.9));
students.add(new Student(“21e”,“123”,22.2));
students.add(new Student(“qwq”,“wqqe”,455.4));
System.out.println(students);
}
}
运行结果:
public static void main(String[] args) {
ArrayList integers = new ArrayList<>();
integers.add(11);
integers.add(2);
integers.add(13);
Collections.sort(integers);
System.out.println(integers);
}
运行结果:
public static void main(String[] args) {
String str1 = “welcome to bit”;
String str2 = “come”;
ArrayList list1 = new ArrayList<>();
for (int i = 0; i < str1.length(); i++) {
char ch = str1.charAt(i);
if(!str2.contains(ch+“”)){
list1.add(ch);
}
}
for (char ch : list1 ) {
System.out.print(ch);
}
}
运行结果:
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
class Card{
private int rank;
private String suit;
public Card(int rank, String suit) {
this.rank = rank;
this.suit = suit;
}
public int getRank() {
return rank;
}
public void setRank(int rank) {
this.rank = rank;
}
public String getSuit() {
return suit;
}
public void setSuit(String suit) {
this.suit = suit;
}
@Override
public String toString() {
return “{” + suit + “->” +rank + “}”;
}
}
public class Test {
private static final String[] suits = {“❤”,“♠”,“♣”,“♦”};
/**
构造一副牌
@return
*/
public static List buyCard(){
ArrayList cards = new ArrayList<>();
//四种花色
for (int i = 0; i < 4; i++) {
//每个花色13张牌
for (int j = 0; j < 13; j++) {
/*String suit = suits[i];//花色
int rank = j ;//牌数
Card card = new Card(rank,suit);//得到一张牌card
cards.add(card);*/
cards.add(new Card(j,suits[i]));//优化后的代码
}
}
return cards;
}
private static void swap(List cards,int i,int j){
Card tmp = cards.get(i);
cards.set(i,cards.get(j));
cards.set(j,tmp);
}
/**
洗牌
@param cards
*/
public static void shuffle(List cards){
int size = cards.size();
for (int i = size - 1; i > 0; i–) {
Random random = new Random();
int rand = random.nextInt(i);
swap(cards,i,rand);
}
}
public static void main(String[] args) {
List cards = buyCard();
System.out.println(“刚开始的牌:”+cards);
shuffle(cards);
System.out.println(“洗过后的牌:”+cards);
ArrayList<List> hand = new ArrayList<>();
List hand1 = new ArrayList<>();
List hand2 = new ArrayList<>();
List hand3 = new ArrayList<>();
hand.add(hand1);
hand.add(hand2);
hand.add(hand3);
//每个人,轮流揭牌
for (int i = 0; i < 5; i++) {
for (int j = 0; j < 3; j++) {
Card card = cards.remove(0);//拿一张牌 少一张牌 remov去掉
hand.get(j).add(card);
}
}
System.out.println("第1个人的牌: "+hand1);
System.out.println("第2个人的牌: "+hand2);
System.out.println("第3个人的牌: "+hand3);
System.out.println("剩余的排: "+cards);
}
}
=====================================================================================
package demo;
import java.util.Arrays;
class MyArrayList {
private Object[] elem;//数组
private int usedSize;//代表有效数据个数
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
public MyArrayList() {
this.elem = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
public MyArrayList(int capacity) {
//对参数进行一个判断
if (capacity > 0) {
this.elem = new Object[capacity];
} else if (capacity == 0) {
this.elem = new Object[0];
} else {
throw new IllegalArgumentException(“不能为负数!”);
}
}
/**
尾插 e
@param e
@return
*/
public boolean add(E e) {
//缺点一个真正的容量.扩容
ensureCapacityInternal(usedSize + 1);
elem[usedSize] = e;
usedSize++;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
//计算出需要的容量
int capacity = calculateCapacity(elem, minCapacity);
//拿着计算出的容量,去判断是否扩容,需要扩容就扩容
ensureExplicitCapacity(capacity);
}
private static int calculateCapacity(Object[] elem, int minCapacity) {
//1.是否elem数组分配过大小
if (elem == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(10, minCapacity);
}
//2.分配过 就返回+1的值
return minCapacity;
}
private void ensureExplicitCapacity(int minCapacity) {
if (minCapacity - elem.length > 0) {
//扩容
grow(minCapacity);
}
}
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
private void grow(int minCapacity) {
int oldCapacity = elem.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);//1.5倍扩容
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
//说明需要的容量非常大
newCapacity = hugeCapacity(minCapacity);
elem = Arrays.copyOf(elem, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0)
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
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