For loop with a generic array?

问题

Going back over my basic ADT stuff here, and trying to kill two birds with one stone by learning Java while I amTrying to write a simple algorithm for a merge sort with a generic linked list ( which I am creating myself). It's proving to be far more difficult than I had first imagined ! Can anyone help me out please ? I will start out working on the basics and will update this post as I get further in.

My code for the generic linked list is as follows :

    public class NodeList<T> {
  private Comparable head;
  private NodeList tail;
  public NodeList( Comparable item, NodeList list ) {
    head = item;
    tail = list;
  }

}

I am trying to access this class in another class I have made, which is as follows :

public class MyList<T> {

  private NodeList<T> nodes;
  private int size;
  public MyList( ) { 
    nodes = null; 
  }

  public MyList(T[] array ){
    for(int countArray = 0; countArray <= array.length() ; countArray++) {
      nodes= new NodeList( value, nodes );
      size++;
    }
  }

which should add generic items from an array, using a linked list. Unfortunately, it doesn't and this is the first problem I have encountered. I am getting the error :

cannot find symbol : method length().

Can someone give me some advice on how I could fix this?

Many thanks!

回答1:

on the array you don't have a length() method but a length member : array.length

Additionally, you'll want to stop iterating before countArray reaches array.length and initialise size before using it:

final int arrayLength = array.length;
size = arrayLength;
nodes = null;
for(int i = 0; i < arrayLength; ++i) {
      nodes = new NodeList(array[i], nodes);
}

nodes = null;
size = array.length;
for(T element : array) {
      nodes = new NodeList(element, nodes);
}

回答2:

The method on a collections class is .size(), or on an array it is the .length property.

But you can loop through either of these with an "enhanced" for loop (aka foreach):

for( T element : array ) {
    nodes = new NodeList( value, nodes );
    size++;
}

回答3:

In addition to what others have posted, you might also want to use your generic parameter T:

public class NodeList<T> {
  private T head;
  private NodeList<T> tail;
  public NodeList( T item, NodeList list ) {
    head = item;
    tail = list;
  }
}

回答4:

length is a field, not a method, on arrays. Remove the parentheses.

for(int countArray = 0; countArray <= array.length ; countArray++) {
  nodes= new NodeList( value, nodes );
  size++;
}

Here's a better way to write the whole constructor:

public MyList(T[] array ){
    nodes = null;
    for(T t : array) {
        nodes = new NodeList(t, nodes);
    }
    size = array.length;
}

回答5:

If you want to make sure that only comparable items are possible:

public class NodeList<T extends Comparable<T> > {
  private T head;
  private NodeList<T> tail;
  public NodeList( T item, NodeList<T> list ) {
    head = item;
   tail = list;
  }
}

And

public class MyList<T extends Comparable<T>> {
...
}

Additionally, if your constructor uses var args, you get a more convenient way of creating a list:

public MyList(T... array ) {
  for( T item : array ) {
    nodes = new NodeList<T>(item, nodes); 
  }
  size = array.length;
}

That way you can call the constructor as follows:

new MyList<Long>(); //empty list
new MyList<Long>( 1L ); //one entry
new MyList<Long>( 1L, 2L, 3L ); //3 entries
Long[] array = new Long[] { 1L, 2L, 3L, 4L };
new MyList<Long>( array ); //use existing array

回答6:

It is array.length not array.length().

for(int countArray = 0; countArray <= array.length ; countArray++) {

will resolve your compilation error.

来源：https://stackoverflow.com/questions/5183220/for-loop-with-a-generic-array

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