如何修复：java.lang.ClassCastException：java.util.ArrayList无法强制转换为java.lang.Integer

Question

I encountered a problem when I tried to run the following code:

ArrayList paretoSet=new ArrayList();   // contains a collection of ArrayList
ArrayList<Integer> toPass=new ArrayList<Integer>();
int[] fParetoSet=new int[ParetoSet.size()];
int[] gParetoSet=new int[ParetoSet.size()];

for (int i=0;i<paretoSet.size();i++){
        toPass.clear();
        toPass.add((Integer)paretoSet.get(i));
        int [] totake=calculate(toPass);
        fParetoSet[i]=totake[0];
        gParetoSet[i]=totake[1];       
    }

` where claculate(ArrayList x) is a method that takes an integer arraylist and returns an integer array. I can not make Paretoset an integer arraylist as it creates problem in other parts of my program. I encountered an exception in the line toPass.add((Integer)paretoSet.get(i));
as java.lang.ClassCastException: java.util.ArrayList cannot be cast to java.lang.Integer How should I fix this problem?
Thanks in advance

Answer 1

If ParetoSet is a collection of ArrayList , then the call ParetoSet.get(i) will return the ArrayList at index i . As the error says, an ArrayList is not aa type of Integer and cannot be cast to one.

Other points of interest:

• your variable should be camel case: pareToSet
• auto-boxing means the cast in your for loop has been unnecessary since JDK5
• Paretoset has been declared with a raw type
• type inference makes new ArrayList<Integer>() redundant since JDK7

EDIT

• your variable should be camel case: paretoSet , as per Jim's comments

EDIT

An ArrayList is neither conceptually, or actually an Integer. If you say the sentence 'A list is a type of integer' it doesn't make sense. If we check the javadoc for ArrayList we can see that its inheritance tree is:

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractList<E>
java.util.ArrayList<E>

All Implemented Interfaces:
Serializable, Cloneable, Iterable<E>, Collection<E>, List<E>, RandomAccess

So we can say for example that ArrayList is a type of AbstractList , or AbstractCollection , but not an Integer as Integer is not part of its lineage.

Answer 2

your program having typecasting issue on below lines. your are trying to add typecast list of arraylist to Integer arryalist. this wrong. if you want add the arraylist objects to some other arryalist. Please use generics as object.

ArrayList ParetoSet=new ArrayList();   // contains a collection of ArrayList
toPass.add((Integer)ParetoSet.get(i));

This should be like below:

ArrayList<ArrayList> ParetoSet=new ArrayList<ArrayList>();  
ArrayList<ArrayList> toPass=new ArrayList<ArrayList>();  
toPass.add(ParetoSet.get(i));

Then you should change code bit to fit your further your logics