如何制作更大的数组并复制java中前一个数组的信息？

I'm trying to use to store info from a text file into a array, but the project asks us to do so by making a temporary array and increasing its size as we read more files to accomadate for the new objects. How can I do this?

我正在尝试使用将文本文件中的信息存储到数组中,但项目要求我们这样做,方法是创建一个临时数组并增加其大小,因为我们会读取更多文件以适应新对象。我怎样才能做到这一点?

/**
 * reads a csv data file and returns an array of acquaintances
 * @param path File path of CSV file
 * @return Acquaintances from the file
 */
public static Acquaintance[] read (String path)  {
    //create an array of acquaintances 
    Acquaintance[] acqs = new Acquaintance[0];

    //open the file
    try {
        BufferedReader file = new BufferedReader(new FileReader(path));

        //read the file until the end
        String line;

        while( (line = file.readLine()) != null) {
            // parse the line just read
            String[] parts = line.split(",");

            //create an acquaintance object
            Acquaintance a = new Acquaintance(parts[0], parts[1], Double.parseDouble(parts[2]));

            acqs[0] = a;

            //Add the object to the array

            //(1) create a new Acquaintance array, with one extra element
            Acquaintance[] tmp = new Acquaintance[acqs.length+1];

            //(2) copy all old elements into new
            Acquaintance[] tmp = acqs.clone();

            //(3) assign new Acquaintance object to last element of the array

            //(4) assign new array's address to acqs
            //for loop
        }

3 个解决方案

#1

If your teacher requires a specific method you should probably have listened more closely in class :-)

如果您的老师需要特定的方法,您可能应该在课堂上更密切地倾听:-)

You can grow an array by one like this:

您可以像这样增加一个数组:

myArray = Arrays.copyOf(myArray,myArray.length+1);

but there is no for-loop to be seen. You can do it using older Java methods like this:

但是没有可以看到的循环。你可以使用像这样的旧Java方法来做到这一点:

Object[] tmpArray = new Object[myArray.length+1];
System.arraycopy(myArray,0,tmpArray,0,myArray.length);
myArray = tmpArray;

Again, no for-loop required. The only advantage is that it runs with Java 1.5. The only "need" for a for-loop in this question is to do what System.arraycopy (which is used by Arrays.copyOf) does more efficiently. Like this:

同样,不需要for-loop。唯一的优点是它运行Java 1.5。在这个问题中for循环的唯一“需要”是做System.arraycopy(由Arrays.copyOf使用)更有效地做什么。像这样:

Object[] tmpArray = new Object[myArray.length+1];
for(int i=0;i<myArray.length;i++) tmpArray[i]=myArray[i];
myArray = tmpArray;

The idea is thus to give you practice in using for-loops, not to actually solve the problem in a good way.

因此,这个想法是让你练习使用for循环,而不是以一种好的方式实际解决问题。

The easiest way to grow an array as you use it is to use some form of List, java.util.ArrayList for example, let the list handle storing the data, and invoke the List's toArray() method when you are finished.

在使用数组时增长数组的最简单方法是使用某种形式的List,例如java.util.ArrayList,让列表句柄存储数据,并在完成后调用List的toArray()方法。

#2

Use

Arrays.copyOfRange(T[] original,int from, int to)

method to copy one array to diffrent array

将一个数组复制到不同数组的方法

#3

I'm going to ignore the fact that the requirement to use arrays for this makes no sense whatsoever. What you want to do is assign your initial array a given size, and increase it by a set amount whenever you reach the end of the storage space. Typically, you would increase the underlying storage by a factor of two, whenever you run out of space. Heres some pseudocode:

我将忽略这样一个事实,即为此使用数组的要求毫无意义。您要做的是将初始数组指定给定大小,并在到达存储空间末尾时将其增加一定量。通常,只要空间不足,就可以将基础存储增加两倍。下面是一些假代码:

public Acquaintance[] getAcquaitances(final String path) {
    if(path == null) return null;

    int count = 0, initialSize = 16;
    Acquaintance[] temporaryArr = new Acquaintance[initialSize];

    // Open file here

    String line = null;
    while((line = file.readLine()) != null) {
        if(count >= temporaryArr.length) {
            Acquaintance[] switch = temporaryArr;
            temporaryArr = new Acquaintance[switch.length * 2] // increase size by factor of 2
            for(int i = 0; i < switch.length; i++) {
                temporaryArr[i] = switch[i];
            }
         }

         temporaryArr[count] = createAcquantainceFromLine(line);

         ++count;
     }

     Acquaintance[] results = new Acquaintance[count];
     for(int i = 0; i < count; i++) {
         results[i] = temporaryArr[i];
     }

     return results;
}

With some slight modifications, the above code should work for you. Study it, embed it, but then totally ignore it and if you ever have a future need for expanding arrays please use ArrayList instead. Or System.arraycopy.

稍作修改,上面的代码应该适合您。研究它,嵌入它,但然后完全忽略它,如果你将来需要扩展数组,请使用ArrayList。或System.arraycopy。

#1