Error with code, please help.

I'm stuck. If anyone could help out and explain it would be appreciated.

./com/teamtreehouse/Treet.java:6: error: cannot find symbol
import java.io.ArrayList;
^
symbol: class ArrayList
location: package java.io
./com/teamtreehouse/Treet.java:7: error: cannot find symbol
import java.io.Arrays;
^
symbol: class Arrays
location: package java.io
./com/teamtreehouse/Treet.java:56: error: cannot find symbol
return Arrays.asList(words);
^
symbol: variable Arrays
location: class Treet
./com/teamtreehouse/Treet.java:68: error: cannot find symbol
List<String> results = new ArrayList<String>();
^
symbol: class ArrayList
location: class Treet
4 errors

package com.teamtreehouse;

import java.io.Serializable; import java.io.ArrayList; import java.io.Arrays; import java.util.Date; import java.util.List;

public class Treet implements Comparable<Treet>, Serializable { private static final long serialVersionUID = 7146681148113043748L; private String mAuthor; private String mDescription; private Date mCreationDate;

public Treet(String author, String description, Date creationDate) { mAuthor = author; mDescription = description; mCreationDate = creationDate; }

@Override public String toString() { return String.format("Treet: \"%s\" by %s on %s", mDescription, mAuthor, mCreationDate); }

@Override public int compareTo(Treet other) {

if (equals(other)) {
  return 0;
}
int dateCmp = mCreationDate.compareTo(other.mCreationDate);
if (dateCmp == 0) {
  return mDescription.compareTo(other.mDescription);
}
return dateCmp;

}

public String getAuthor() { return mAuthor; }

public String getDescription() { return mDescription; }

public Date getCreationDate() { return mCreationDate; }

public List<String> getWords() { String[] words = mDescription.toLowerCase().split("[^\w#@']+"); return Arrays.asList(words);
}

public List<String> getHashTags(){ return getWordsPrefixedWith("#"); }

public List<String> getMentions(){ return getWordsPrefixedWith("@"); }

private List<String> getWordsPrefixedWith(String prefix){ List<String> results = new ArrayList<String>(); for (String word : getWords()){ if (word.startsWith(prefix)){ results.add(word); } } return results; }

}