我正在努力学习java,并且我试图理解如何从一个类调用函数到另一个类。我有两个类(class),我将在下面发布。当我在 ToDo 类中调用 H2db.addItems() 以及在 H2db 类中调用 ToDo.menu() 时,我看到“无法从静态上下文引用非静态方法”。谷歌搜索显示它与拥有实例有关(?),但我不确定这意味着什么。
类待办事项:
package com.company;
import java.io.*;
import java.lang.reflect.Method;
import java.sql.*;
import java.util.ArrayList;
import java.util.Scanner;
public class ToDo {
//BufferedReader inputread = new BufferedReader(new InputStreamReader(System.in));
Scanner inputread = new Scanner(System.in);
ArrayList<String> toDoList = new ArrayList<String>();
public void menu() {
clearConsole();
System.out.println("Welcome to the To-Do program.");
System.out.println();
System.out.println();
System.out.println("Please select an option from the following menu, using the number.:");
System.out.println("1- View To-Do List");
System.out.println("2- Add Item To List");
System.out.println("3- Remove Item From List");
System.out.println("4- Save List");
System.out.println("5- Load List");
int userinput = inputread.nextInt();
switch (userinput) {
case 1:
clearConsole();
displayList(true);
menu();
break;
case 2:
clearConsole();
H2db.addItems();
break;
case 3:
clearConsole();
deleteItem();
break;
case 4:
clearConsole();
try {
saveList();
}
catch(IOException e){
return;
};
case 5:
clearConsole();
try {
loadList();
}
catch(IOException e){
return;
};
}
}
public void clearConsole() {
for (int i = 0; i < 25; i++) {
System.out.println();
}
}
public void addItem() {
System.out.println("Please type the item to add to the To-Do List");
//The skip gets over the leftover newline character
inputread.skip("\n");
String newItem = inputread.nextLine();
toDoList.add(newItem);
System.out.println("Your item has been added! Type any key and press Enter to continue");
String discardMe = inputread.next();
menu();
}
public void displayList(boolean finish) {
if (toDoList.isEmpty()) {
System.out.println("Add an activity.");
} else {
for (String listItem: toDoList) {
System.out.println(listItem);
}
}
if (finish) {
System.out.println();
System.out.println("This is your list. Type any key and press Enter to continue");
String discardMe = inputread.next();
}
}
public void deleteItem() {
if (toDoList.isEmpty()) {
System.out.println("For fuck's sake, add an activity.");
} else {
System.out.println("Please choose the number of the line you want to delete:");
displayList(false);
int userinput = inputread.nextInt();
int listPos = userinput - 1;
toDoList.remove(listPos);
System.out.println("That item has been deleted. Type any key and press Enter to continue.");
String discardMe = inputread.next();
}
menu();
}
public void saveList() throws IOException {
System.out.println("Saving list...");
PrintWriter writer = new PrintWriter("list.txt", "UTF-8");
for (String listItem : toDoList) {
writer.println(listItem);
}
writer.close();
System.out.println("List saved! Type any key and press Enter to continue");
String discardMe = inputread.next();
menu();
}
public void loadList() throws IOException {
System.out.println("Loading list...");
File loadFile = new File("list.txt");
Scanner scanner = new Scanner(loadFile);
while (scanner.hasNext()) {
String item = scanner.nextLine();
toDoList.add(item);
}
System.out.println("List loaded! Type any key and press Enter to continue");
String discardMe = inputread.next();
menu();
scanner.close();
}
}
H2db 类:
package com.company;
import java.sql.*;
import java.util.ArrayList;
import java.util.Scanner;
public class H2db {
Scanner inputread = new Scanner(System.in);
ArrayList<String> toDoList = new ArrayList<String>();
public void connect(){
try {
Class.forName("org.h2.Driver");
Connection conn = DriverManager.getConnection("jdbc:h2:~/toDo", "user", "");
Statement stat = conn.createStatement();
stat.execute("CREATE TABLE IF NOT EXIST 'todo'(id int NOT NULL AUTO_INCREMENT primary key, item varchar(255))");
}
catch( Exception e) {
System.out.println(e.getMessage());
}
}
public void addItems() {
PreparedStatement addstat;
try {
Class.forName("org.h2.Driver");
Connection conn = DriverManager.getConnection("jdbc:h2:~/toDo", "user", "");
System.out.println("Please type the item to add to the To-Do List");
inputread.skip("\n");
String newItem = inputread.nextLine();
addstat = conn.prepareStatement("INSERT INTO toDo (item)" +"VALUES (?)");
addstat.setString(1, newItem);
addstat.executeUpdate();
System.out.println("Your item has been added! Type any key and press Enter to continue");
String discardMe = inputread.next();
ToDo.menu();
}
catch( Exception e) {
System.out.println(e.getMessage());
}
}
}
任何帮助都会很棒,谢谢。我知道这只是知识上的差距,但我似乎无法理解这个概念叫什么。
最佳答案
我认为您缺少面向对象编程的基础。 首先问问自己:ToDo 类代表哪个对象?查看您的代码,它是一个 GUIManager。
要正常工作,GUIManager 需要另一个能够在名为 H2DBManager 的数据库(当前为 H2db 类)中存储数据的对象。 这意味着 GUIManager 必须在其构造函数中实例化一个 H2DBManager,即 h2DBManager,并在需要时使用其常规方法(非静态)。当 H2DBManager 类完成任务(即已完成在方法 addItems 中存储某些内容)时,控制流自动返回到 GUIManager 类,因此您永远不需要从 H2DBManager 类显式调用 GUIManager 操作(静态或非静态)。
关于java - 在这种情况下,静态函数和实例如何协同工作?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/28297550/