從列表中的對象的屬性創建列表

Question

我希望我的標題能夠更簡潔，但我不太清楚如何描述我想要做的事情。我有一個列表，其中包含我創建的一個類的一些對象。每個對象都有一些屬性，可以被一些getter和setter訪問。我想爲每個屬性創建一個列表，獲取每個對象的屬性並將它們放入它們各自的列表中。

我目前做這與下面的代碼：

ArrayList<Integer> counts = new ArrayList<Integer>(); 
ArrayList<String> colors = new ArrayList<String>(); 
ArrayList<String> shapes = new ArrayList<String>(); 
ArrayList<String> fills = new ArrayList<String>(); 

for (Card card: cards) { 
    counts.add(card.getCount()); 
    colors.add(card.getColor()); 
    shapes.add(card.getShape()); 
    fills.add(card.getFill()); 
} 

但我不知道是否有一個更短的，也許更好，方式做到這一點？或者我是否已經做到了「正確」？謝謝！

Answer 1

你的方式看起來不錯。如果您使用Java 8 Streams，則可以使用較少的代碼行編寫代碼，但這需要在cards列表上執行4次迭代，而不是您當前擁有的單次迭代。

例如，創建計數的列表，你可以寫：

List<Integer> counts = cards.stream().map(Card::getCount).collect(Collectors.toList()); 

Answer 2

如果你正在開發一個set game，這裏是檢查並完成卡套的解決方案：

卡。 Java的：

import java.util.Locale; 

public final class Card { 
    public static void main(String[] args) { 
     Card a = new Card(Count.ONE, Color.RED, Fill.STRIPED, Shape.DIAMOND); 
     Card b = new Card(Count.TWO, Color.RED, Fill.SOLID, Shape.DIAMOND); 
     Card c = completeSet(a, b); 
     System.out.println(c); 
    } 

    public int count; 
    public int color; 
    public int fill; 
    public int shape; 

    public Card() { 

    } 

    public Card(Count count, Color color, Fill fill, Shape shape) { 
     setCount(count); 
     setColor(color); 
     setFill(fill); 
     setShape(shape); 
    } 

    // getters and setters to operate with the attribute enumerations 
    public Count getCount() { 
     return Count.values()[count]; 
    } 

    public Color getColor() { 
     return Color.values()[color]; 
    } 

    public Shape getShape() { 
     return Shape.values()[shape]; 
    } 

    public Fill getFill() { 
     return Fill.values()[fill]; 
    } 

    public void setCount(Count count) { 
     this.count = count.ordinal(); 
    } 

    public void setColor(Color color) { 
     this.color = color.ordinal(); 
    } 

    public void setShape(Shape shape) { 
     this.shape = shape.ordinal(); 
    } 

    public void setFill(Fill fill) { 
     this.fill = fill.ordinal(); 
    } 

    public static int completeAttribute(int a, int b) { 
     if (a == b) { 
      // attribute for each same 
      return a; 
     } else { 
      // attribute for each different 
      int c; 
      for (c = 0; c < 3; c++) { 
       if (c != a && c != b) { 
        break; 
       } 
      } 
      return c; 
     } 
    } 

    public static Card completeSet(Card a, Card b) { 
     // determine missing card to make a set 
     Card result = new Card(); 
     result.count = completeAttribute(a.count, b.count); 
     result.color = completeAttribute(a.color, b.color); 
     result.shape = completeAttribute(a.shape, b.shape); 
     result.fill = completeAttribute(a.fill, b.fill); 
     return result; 
    } 

    public static boolean isSet(Card a, Card b, Card c) { 
     // check if it is a set by completing it 
     return completeSet(a, b).equals(c); 
    } 

    @Override 
    public boolean equals(Object that) { 
     // currently unused 
     if (this == that) { 
      return true; 
     } 
     return that != null && that instanceof Card && this.equals((Card) that); 
    } 

    public boolean equals(Card that) { 
     if (this == that) { 
      return true; 
     } 
     return that != null 
       && this.color == that.color 
       && this.count == that.count 
       && this.fill == that.fill 
       && this.shape == that.shape; 

    } 

    @Override 
    public int hashCode() { 
     // currently unused 
     int result = count; 
     result = 31 * result + color; 
     result = 31 * result + shape; 
     result = 31 * result + fill; 
     return result; 
    } 

    @Override 
    public String toString() { 
     // pretty print attributes 
     String result = getCount() + " " + getColor() + " " + getFill() + " " + getShape(); 
     result = result.toLowerCase(Locale.ROOT); 
     if(getCount() != Count.ONE) { 
      result += "s"; 
     } 
     return result; 
    } 

    // enumerations for pretty names 
    public enum Count { 
     ONE, 
     TWO, 
     THREE 
    } 

    public enum Color { 
     RED, 
     GREEN, 
     VIOLET 
    } 

    public enum Shape { 
     ELLIPSE, 
     DIAMOND, 
     TWIRL 
    } 

    public enum Fill { 
     OPEN, 
     STRIPED, 
     SOLID 
    } 
} 

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輸出：three red open diamonds