Java多维数组列表

Question

我只是写了这个函数来添加值到多维数组列表，但不明白为什么所有的增加值覆盖到最近的增加值。我得弄清楚为什么会这样。正如javadoc所说，它必须这样做"将指定的元素附加到该列表的末尾"。我在他的代码中没有看到任何地方，以前的附加价值可以被覆盖。还有一件奇怪的事情是数组的大小会增加

下面是我的输出:

[0.4, 0.42, 0.4, 1.2200000000000002, 0.0010000000000000009] after add 0 [0.4, 0.42, 0.4, 1.2200000000000002, 0.0010000000000000009] after add 1 [0.4, 0.42, 0.4, 1.2200000000000002, 0.0010000000000000009] after add 2 [0.4, 0.42, 0.4, 1.2200000000000002, 0.0010000000000000009] after add 3 [0.4, 0.42, 0.4, 1.2200000000000002, 0.0010000000000000009] after add 4 [0.4, 0.42, 0.42, 1.24, 0.01200000000000001] after add 0 [0.4, 0.42, 0.42, 1.24, 0.01200000000000001] after add 1 [0.4, 0.42, 0.42, 1.24, 0.01200000000000001] after add 2 [0.4, 0.42, 0.42, 1.24, 0.01200000000000001] after add 3 [0.4, 0.42, 0.42, 1.24, 0.01200000000000001] after add 4 [0.4, 0.42, 0.42, 1.24, 0.01200000000000001] after add 5 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 0 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 1 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 2 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 3 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 4 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 5 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 6

下面是我的函数:

public void createArrayList(ArrayList<Double> list,ArrayList<Double>
listLaid,int size,double sum, double absValue){   
ArrayList<Double> workArray = new ArrayList<>();  
ArrayList<ArrayList<Double>>
workMatrix= new ArrayList<>();
double abs=0;
for(int a = 0; a < size ;a++){
     for(int b = 0; b < size ; b++){
           for (int c = 0 ; c < size; c++){
             abs =Math.abs(sum - list.get(a) - list.get(b) - list.get(c));
                 if( abs <= absValue){
                       double totalSum = listLaid.get(a) + listLaid.get(b) +                                            
                                         listLaid.get(c);
                       workArray.clear();
                       workArray.add(listLaid.get(a));
                       workArray.add(listLaid.get(b));
                       workArray.add(listLaid.get(c));
                       workArray.add(totalSum);
                       workArray.add(abs);
                       workMatrix.add(workArray);
                  }
              }
         } 
    }                   
} 
for (int i = 0; i < workMatrix.size(); i++)   
System.out.println(darbineMat.get(i)+"  after add  "+i);

Answer 1

您总是在列表中添加相同的实例。这意味着最终workMatrix的所有行都相同。

您必须为每个新行创建一个新的ArrayList。替换

workArray.clear();

workArray = new ArrayList<Double>();

应该足够了。

---

EDIT:"引用"的概念乍一看可能会令人困惑(在某些编程语言中，这个概念并不以同样的方式存在)。但是当你创建一个像这样的列表实例时

ArrayList<Double> workArray = new ArrayList<Double>();  

那么workArray只是对您正在创建的实例的引用。你可以把它想象成一个"句柄"(事实上，它更像是C/c++中的一个"指针"，但这现在可能变得太混乱了)。关键是当你写

时

ArrayList<ArrayList<Double>> workMatrix = new ArrayList<>();
ArrayList<Double> workArray = new ArrayList<>();  
workMatrix.add(workArray);

则只将引用添加到workMatrix列表中。这个引用仍然指向您创建的唯一实例:

// This prints "true": The first element of the matrix
// and the workArray are IDENTICAL. (Not only equal, 
// but really IDENTICAL)
System.out.println(workMatrix.get(0) == workArray);