ArrayList值索引获取及Xamarin转Java代码报错问题求助

Hey there! Let's tackle your two issues one by one—learning a new language like Java after Xamarin can feel tricky at first, but we'll work through this together.

Fixing Conversion-Related Red Line Errors #

Since you haven't shared specific error messages or code snippets, here are some practical, beginner-friendly steps to debug those frustrating red lines:

• Read the error details closely: Hover over the red line in your IDE (like Android Studio) and read the pop-up message. Common issues include:
  • Type mismatches: Xamarin's .NET types (e.g., DateTime) don't map directly to Java equivalents (like LocalDateTime), or method return types don't align.
  • Missing imports: Java requires explicit imports for most classes (e.g., import java.util.ArrayList;), which Xamarin handles automatically behind the scenes.
  • Method name differences: Xamarin uses PascalCase (e.g., Add()) while Java uses camelCase (e.g., add()), and some methods have entirely different names (like Xamarin's List.Contains() vs Java's ArrayList.contains()).
• Compare API equivalents side-by-side: Look up the Xamarin method/class you're converting and find its Java counterpart. For quick reference:
  • Xamarin's List<T> → Java's ArrayList<T>
  • Xamarin's String.IsNullOrEmpty(str) → Java's str == null || str.isEmpty()
• Test small code chunks first: Don't convert an entire file at once. Start with a single method or code block, get it to compile, then move on. This makes it way easier to pinpoint where things go wrong.
• Leverage IDE suggestions: Most Java IDEs have a lightbulb icon next to errors—click it to see auto-fix suggestions (like adding missing imports, converting types, or correcting method names). This is a total lifesaver for beginners!

Getting an Element's Index in ArrayList #

There are two common scenarios here, depending on whether you're looking for a single match or all matches:

1. Find the first occurrence of an element #

Use the indexOf(Object o) method—it returns the index of the first matching element, or -1 if the element isn't in the list. Example:

// Create an ArrayList of strings
ArrayList<String> snacks = new ArrayList<>();
snacks.add("Chips");
snacks.add("Cookies");
snacks.add("Popcorn");

// Get index of "Cookies"
int cookieIndex = snacks.indexOf("Cookies"); // Returns 1
// Get index of a non-existent element
int candyIndex = snacks.indexOf("Candy"); // Returns -1

2. Find all indices of a repeated element #

If your list has duplicate values and you need all their indices, loop through the list manually and collect matches:

ArrayList<Integer> scores = new ArrayList<>();
scores.add(90);
scores.add(85);
scores.add(90);
scores.add(78);
scores.add(90);

// Collect all indices where the score is 90
List<Integer> highScoreIndices = new ArrayList<>();
for (int i = 0; i < scores.size(); i++) {
    if (scores.get(i).equals(90)) {
        highScoreIndices.add(i);
    }
}
// highScoreIndices will contain [0, 2, 4]

Important note for custom objects #

If you're using a custom class (like Person or Product) in your ArrayList, you must override the equals() and hashCode() methods in your class. Otherwise, indexOf() will compare object references instead of the actual content of the objects, which won't work as expected. For example:

class Person {
    private String name;
    private int age;

    // Constructor, getters, setters...

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Person person = (Person) o;
        return age == person.age && Objects.equals(name, person.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }
}

内容的提问来源于stack exchange，提问作者Alsong Dun Dunstan Philander