比较同一List Java中的元素
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Comparing elements in the same List Java
提问by Anton James
I want to compare the elements in the same List (The List "list)and if they are the same print out a line, I'm fairly new to programming and I dont yet completely understand the differencebetween Listand an arrayand when you use them.
我想比较相同的元素，List (The List "list)如果它们相同，则打印出一行，我对编程还很陌生，我还没有完全理解List和数组之间的区别以及何时使用它们。
Someone kindly suggested the code below and although it compliles nothing is being printed out. Since there are two pairs in my array I would expect something to print out at least once. Any help would be most appreciated thanks!
有人好心地建议了下面的代码，虽然它编译了但没有打印出来。由于我的数组中有两对，我希望至少打印一次。任何帮助将不胜感激谢谢！
List<Double> list = new ArrayList<Double>();
while (search.find()){
list.add(Double.parseDouble(search.group(1)));
}
for(int i= 0 ; i < list.size()-1; i++){
//System.out.println(list.get(i)); // Successfully prints out all doubles in the SHADING:BUILDING:DETAILED class
for(int k = i+1 ; k < list.size() ; k++){
if(list.get(i) == list.get(k)){
System.out.println(i + "and" + k + "are pairs");
}
}
}
}
}
采纳答案by fasaas
The way you have coded the comparation repeats itself. Besides, that is not the correct way you access an element in a list, I'm afraid.
您对比较进行编码的方式会重复。此外，恐怕这不是您访问列表中元素的正确方式。
For example, lets say you have the i = 0 (value 2.0 for example) and k = 2 (value 5.2), but later you have i = 2 (5.2) and k = 0 (2.0), which is the same. I suggest you the following, which avoids this repetition.
例如，假设您有 i = 0（例如值 2.0）和 k = 2（值 5.2），但后来您有 i = 2 (5.2) 和 k = 0 (2.0)，这是相同的。我建议你以下，以避免这种重复。
for (int i = 0; i < list.size()-1; i++)
for (int k = i+1; k < list.size(); k++)
if(list.get(i) == list.get(k))
System.out.println(i + " and "+ k +" are pairs");
Of course, you have to make sure that the list's size is bigger than 1, otherwise it will throw OutOfBoundsException. Hope this helps ^^
当然，你必须确保列表的大小大于1，否则会抛出OutOfBoundsException。希望这有帮助^^
EDIT #2 Try and use the following if, it contains a null-safe equals:
编辑 #2 如果它包含一个空安全等于，请尝试使用以下内容：
if(Objects.equals(list.get(i), list.get(k)))
回答by Andrew Vitkus
A list is basically just a variable length array. You can access data from it with the get(int) method which gets the n-th value from the list and set(int, Object) to set the n-th value to the given object.
列表基本上只是一个可变长度的数组。您可以使用 get(int) 方法访问其中的数据，该方法从列表中获取第 n 个值，并使用 set(int, Object) 将第 n 个值设置为给定对象。
The javadocs are your friends. http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html
javadocs 是您的朋友。http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html
回答by alfasin
First, you should fix:
首先，您应该修复：
if(list[i]= list[k]){
To be:
成为：
if(list.get(i) == list.get(k)){
Second, you should add a condition to make sure i != k(or in other words - you're not comparing an element to itself.
其次，您应该添加一个条件来确保i != k（或者换句话说 - 您没有将元素与其自身进行比较。
And third, you can't access a list like you do: list[i]you should use list.get(i)
第三，你不能像你那样访问列表：list[i]你应该使用list.get(i)
回答by David Navarro Astudillo
You can use the following code:
您可以使用以下代码：
public void compareElementToList() {
List<Double> list = Arrays.asList(1.0, 1.2, 1.3, 1.4, 1.0);
for (int i = 0; i < list.size(); i++) {
Double itemI = list.get(i);
for (int j = 0; j < list.size(); j++) {
Double itemJ = list.get(j);
if (i != j) {
if (itemI.compareTo(itemJ) == 0) {
System.out.println("Element " + i + " and " + j + " are pairs.");
} else {
System.out.println("There are no even elements.");
}
} else {
System.out.println("Element omited.");
}
}
}
}
