java - Java 错误和语法教程

Question

我正在寻求有关自助的帮助，这有点矛盾。我如何通过解决更多我自己的问题来减少对你们好人的骚扰？

我在 Java 编程的最后一周，我在学习 Java 方面遇到了巨大的障碍。我读过所有的书，但我总是被一些小问题困扰。这就像尝试 build 纸牌屋。我只知道书中显示的语法部分和用法。当我组合事物时，我会遇到可怕的障碍。我尝试了数小时的修补来弄清楚它们。 sun 文档仅显示似乎没有帮助的基本用法

这是我想要的:

当我尝试做某事但它不像下面的数组列表操作那样工作时，我想找到一个地方或程序来显示示例代码，例如向 arrayList 添加一个额外的类实例。我在哪里可以简明地了解这一点，而不必为每个语法错误问一个或两个问题？ Google for Java 在哪里？是否有程序可以接受您的错误并向您展示如何修复它们(或提供建议)？

/tmp/jc_4083/Inventory.java:101: incompatible types
found   : RatedDVD[]
required: java.util.ArrayList
        dvdlist = temp;
                  ^
/tmp/jc_4083/Inventory.java:110: array required, but java.util.ArrayList found
            if (p != dvdlist[i]) {
                            ^
/tmp/jc_4083/Inventory.java:111: array required, but java.util.ArrayList found
                temp[i-adj] = dvdlist[i];
                                     ^
/tmp/jc_4083/Inventory.java:115: incompatible types
found   : RatedDVD[]
required: java.util.ArrayList
        dvdlist = temp;

如果有人有兴趣帮我看一下，这是我为这门课编写的代码:

//Contruct inv and allow for methods add, get, size, sort, and value
import java.util.*;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;

public class Inventory
{// class Inventory
    private ArrayList<RatedDVD> dvdlist;// declare dvdlist as ArrayList of RatedDVD
    private int numDVDs;

    public Inventory()
    {// method Inventory
        dvdlist = new ArrayList<RatedDVD>();

    }// end method

    // add & get
    public RatedDVD get(int i){return dvdlist.get(i);}// method get

    public void add(DVD d){
    dvdlist = dvdlist d;
    sort();
    }// method add

    public double value()
    {// method value
        double total = 0.0;
        for (int i = 0; i < dvdlist.size(); i++) 
        {// for every pass thru dvdlist add total
        // [DEBUG] consider enhanced for
            total += get(i).feeValue();
        }
        return total;
    }// end method value

    public void sort()
    {// method sort
    // [DEBUG] consider optimization
    int n = dvdlist.size();
        for (int search = 1; search < n; search++) 
        {// for do the following and increment till dvdlist has been searched
            for (int i = 0; i < n-search; i++) 
            {// for step through comparison for entire dvdlist
                if (dvdlist.get(i).getName().compareToIgnoreCase(dvdlist.get(i+1).getName()) > 0) 
                {// if swap necessary then swap
                    RatedDVD temp = dvdlist.get(i);
                    dvdlist.set(i,dvdlist.get(i+1));
                    dvdlist.set(i+1,temp);
                }// end if swap
            }// end for compareto
        }// end outer for
    }// end method sort

    public int size(){return dvdlist.size();}// method size

    public void save() {
        save(true);
    }

    // save it to C:\data\inventory.dat
    public void save(boolean saveagain) {
        try {
            BufferedWriter w = new BufferedWriter(new FileWriter("c:\\data\\inventory.dat"));
            for (int i = 0; i < size(); i++) {
                RatedDVD dvd = get(i);
                w.write( dvd.getItem() + "\n");
                w.write( dvd.getName() + "\n");
                w.write( dvd.getRating() + "\n");
                w.write( dvd.getUnits() + "\n");
                w.write( dvd.getPrice() + "\n");
                w.write( dvd.value() + "\n");
                w.write( dvd.fee() + "\n");
                w.write( dvd.feeValue() + "\n");
                w.newLine();
            }
            // total value of it
            //w.write( value() + "\n");
            w.close();
        } catch (Exception ex) {
            if (saveagain) {
                new File("c:\\data\\").mkdir(); // make file if doesn't exist
                save(false); 
            }
        }
    }

    public int search(String name) {
        for (int i = 0; i < size(); i++) { // check if name string is equal
            if (get(i).getName().equalsIgnoreCase(name)) return i;
        }
        return -1; // we didn't find anything
    }

    // add a new dvd to the end, increasing the array size
    public void add(RatedDVD p) {
        RatedDVD[] temp = new RatedDVD[dvdlist.size()+1];
        for (int i = 0; i < dvdlist.size(); i++) {
            temp[i] = dvdlist[i];
        }
        temp[temp.length-1] = p; // add it at the end
        dvdlist = temp;
    }

    // remove a DVD from the array, and shrink the array size
    public void delete(RatedDVD p) {
        RatedDVD[] temp = new RatedDVD[dvdlist.size()-1];
        int adj = 0;
        for (int i = 0; i < dvdlist.size(); i++) {
            if (p != dvdlist[i]) {
                temp[i-adj] = dvdlist[i];
            }
            else adj = 1;
        }
        dvdlist = temp;
    }
    public int highestNumber() {
        int numb = 0;
        for (int i = 0; i < dvdlist.size(); i++) {
            if (get(i).getItem() > numb) {
                numb = get(i).getItem();
            }
        }
        return numb;
    }   
}// end class inventory

Answer 1

dvdlist 是一个 ArrayList , 它实现了 Collection接口(interface)，而不是数组(顺便说一句，这被称为“接口(interface)程序，而不是实现”原则，您应该将 dvdlist 声明为 java.util.List):

private ArrayList<RatedDVD> dvdlist;// declare dvdlist as ArrayList of RatedDVD

看看 Collection 接口(interface)上的方法，您会发现添加和删除元素所需的一切。

因此，要添加 RatedDVD，您不需要使用 RatedDVD 的临时数组，它无论如何都无法放入 ArrayList 就像你在这里做的那样:

// add a new dvd to the end, increasing the array size
public void add(RatedDVD p) {
    RatedDVD[] temp = new RatedDVD[dvdlist.size()+1];
    for (int i = 0; i < dvdlist.size(); i++) {
            temp[i] = dvdlist[i];
    }
    temp[temp.length-1] = p; // add it at the end
    dvdlist = temp;
}

相反，只需在 dvdlist 上调用 add(Object o) 方法即可。

要删除 RatedDVD 实例，请使用 dvdlist 上的 remove(Object o) 方法。

对于 search() 方法，考虑在 dvdlist 上使用 contains(Object o)。

如果您需要遍历集合，请使用 Iterator :

for (Iterator iter = dvdlist.iterator(); iter.hasNext();) {
   RatedDVD ratedDVD = (RatedDVD) iter.next();
   //rest of the code block removed
}

或者现在使用 Java 5+ 和泛型甚至更快:

for (RatedDVD ratedDVD : dvdlist) {
   // rest of the code here
}

真的，你需要挖掘 Collection Framework .