Java基礎案例教程
阿新 • • 發佈：2019-01-02
靜態程式碼塊
public class Main {
static {
System.out.println("測試類的靜態程式碼執行");
}
public static void main(String[] args){
Person p1=new Person();
Person p2=new Person();
}
}
class Person{
static{
System.out.println("Person類中靜態程式碼塊執行");
}
}
public class Main {
public static void main(String[] args) {
outer.inner in = new outer().new inner();
in.show();
}
}
class outer {
int num = 4;
class inner {
void show() {
System.out.println(num);
}
}
}
內部類
public class 匿名內部類
public class Solution {
public static void main(String[] args){
//定義匿名內部類作為引數傳給AnimalShout()方法
animalShout(new Animal() {
@Override
public void shout() {
System.out.println("喵喵。。。");
}
});
}
private static void animalShout(Animal an) {
an.shout();//呼叫傳入物件的方法
}
}
//定義Animal介面
interface Animal{
void shout();//定義抽象方法
}
String類的初始化
public class Main {
public static void main(String[] args) throws Exception {
// TODO Auto-generated method stub
String str1 = new String();
String str2 = new String("abcd");
char[] charArray = new char[] { 'D', 'E', 'F' };
String str3 = new String(charArray);
System.out.println("a" + str1 + "b");
System.out.println(str2);
System.out.println(str2);
}
}
統計字串的個數
public class Solution {
public static void main(String[] args) {
String str = "nbaernbatnbaynbauinbaopnba";
String key = "nba";
int count = getKeyStringCount(str, key);
System.out.println(count);
}
private static int getKeyStringCount(String str, String key) {
int count = 0;
//if(!str.contains(key))//不包含key
if (str.indexOf(key) == -1)
return count;
int index = 0;//記錄key出現的位置
while ((index = str.indexOf(key)) != -1) {
str = str.substring(index + key.length());
count++;
}
return count;
}
}
定位字串
public class Main {
public static void main(String[] args) throws Exception {
// TODO Auto-generated method stub
String s = "ababcdedcba";
System.out.println(s.length());
System.out.println("first index of 'c':" + s.indexOf('c'));
System.out.println("last index of 'c':" + s.lastIndexOf('c'));
System.out.println("first index of \"ab\":" + s.indexOf("ab"));
System.out.println("last index of \"ab\":" + s.lastIndexOf("ab"));
}
}
字串的替換和去除空格的操作
import java.util.Arrays;
public class Main {
public static void main(String[] args) {
String s = "itcast";
//字串替換操作
System.out.println("將it替換為cn.it:" + s.replace("it", "cn.it"));
//字串去除空格操作
String str = " i t c a s t ";
System.out.println("去除字串兩端的空格：" + str.trim());
System.out.println("去除字串所有的空格：" + str.replace(" ", ""));
}
}
字串判斷操作
import java.util.Arrays;
public class Main {
public static void main(String[] args) {
String s1 = "string", s2 = "str", s3 = "string";
System.out.println("判斷是否以str開頭：" + s1.startsWith("str"));
System.out.println("判斷是否是以ng結尾：" + s1.endsWith("ng"));
System.out.println("判斷是否包含字串abc："+s1.contains("abc"));
System.out.println("判斷兩個字串是否相等："+s2.equals(s1));
}
}
獲取系統全部屬性
import java.util.*;
public class Main {
public static void main(String[] args) throws Exception {
// TODO Auto-generated method stub
//獲取當前系統屬性
Properties properties=System.getProperties();
System.out.println(properties);
System.exit(0);//停止執行
System.exit(1);//非0標識異常退出
//獲取所有系統屬性的key(屬性名)，返回SET物件
Set<String> propertyNames=properties.stringPropertyNames();
for(String key:propertyNames) {
//獲取當前鍵key（屬性名）所對應的值（屬性值）
String value =System.getProperty(key);
System.out.println(key+"------>"+value);
}
}
}
獲取時間和陣列拷貝
import java.util.*;
public class Main {
public static void main(String[] args) throws Exception {
// TODO Auto-generated method stub
long startTime=System.currentTimeMillis();//迴圈開始的當前時間
int sum=0;
for(int i=0;i<(int)1e9;i++) {
sum+=i;
}
long endTime=System.currentTimeMillis();
System.out.println((endTime-startTime)+"毫秒");
int[] fromArray= {10,20,30};
int[] toArray=new int[10];
System.arraycopy(fromArray, 0, toArray, 1, 3);
System.out.println(Arrays.toString(toArray));
}
}
虛擬機器執行時的狀態
import java.util.*;
public class Main {
public static void main(String[] args) throws Exception {
// TODO Auto-generated method stub
Runtime rt = Runtime.getRuntime();//獲取虛擬機器執行的狀態
System.out.println("處理器的個數：" + rt.availableProcessors());
System.out.println("空閒記憶體大小：" + rt.freeMemory() / 1024 / 1024 + "MB");
System.out.println("最大可用記憶體數量：" + rt.maxMemory() / 1024 / 1024 + "MB");
rt.exec("notepad.exe");//命令開啟一個記事本程式
Process process = rt.exec("notepad.exe");//得到表示程序的Process物件
Thread.sleep(3000);//程式休眠3秒
process.destroy();//殺掉程序
}
}
Iterator介面
import java.util.*;
public class Main {
public static void main(String[] args) throws Exception {
ArrayList list = new ArrayList();
list.add("data_1");
list.add("data_2");
list.add("data_3");
//遍歷方法1：Iterator介面
Iterator it = list.iterator();//獲取Iterator物件
while (it.hasNext()) {
Object obj = it.next();
System.out.println(obj);
}
//遍歷方法2:foreach迴圈
/*
foreach(容器中元素型別 臨時變數：容器變數){
執行語句
}
*/
for (Object obj : list) {
System.out.println(obj);
}
}
}
HashSet集合
import java.util.*;
public class Main {
public static void main(String[] args) throws Exception {
HashSet set = new HashSet();
for (int i = 1; i <= 5; i++) {
set.add(i);
set.add(i);//新增重複元素
}
Iterator it = set.iterator();
while (it.hasNext()) {//判斷是否有重複元素
Object obj = it.next();
System.out.println(obj);
}
}
}
HashMap集合
import java.util.*;
public class Main {
public static void main(String[] args) throws Exception {
Map map = new HashMap();//建立Map集合
map.put("1", "Jack");
map.put("2", "Rose");
map.put("3", "Luck");
map.put("3", "Tom");//覆蓋
Set keySet = map.keySet();//獲取鍵的集合
Iterator it = keySet.iterator();//迭代鍵的集合
while (it.hasNext()) {
Object key = it.next();
Object value = map.get(key);//獲取每個鍵所對應的值
System.out.println(key + ": " + value);
}
}
}