从Activity A向Activity B传递Parcelable对象时getExtra返回null

Hey there, let's troubleshoot why your Parcelable A object is showing up as null in Activity B. Based on the code snippet you shared, here are the key fixes and checks to make:

1. Ensure Class B Fully Implements Parcelable #

Since your A class holds a MutableList<B>, B must be a properly implemented Parcelable too. If B's Parcelable logic is broken, createTypedArrayList(B.CREATOR) will return null, which can break the entire deserialization of A.

Here's an example of a correct Parcelable implementation for B:

class B(val someField: String) : Parcelable {
    // Parcel constructor
    constructor(parcel: Parcel) : this(
        parcel.readString() ?: "" // Add default to avoid null issues
    )

    override fun writeToParcel(parcel: Parcel, flags: Int) {
        parcel.writeString(someField)
    }

    override fun describeContents(): Int = 0

    companion object CREATOR : Parcelable.Creator<B> {
        override fun createFromParcel(parcel: Parcel): B = B(parcel)
        override fun newArray(size: Int): Array<B?> = arrayOfNulls(size)
    }
}

2. Verify Intent Passing & Retrieval Code #

Double-check that you're using the same key and correct type handling when sending and receiving the Parcelable:

In Activity A (sending): #

val aInstance = A(1, "Test Name", mutableListOf(B("Sample B")))
val intent = Intent(this, ActivityB::class.java)
intent.putExtra("EXTRA_A_OBJECT", aInstance) // Use a consistent key
startActivity(intent)

In Activity B (receiving): #

// For API 33+, use this type-safe method
val aObject = intent.getParcelableExtra("EXTRA_A_OBJECT", A::class.java)

// For older APIs, use this (ensure no type mismatch)
val aObject = intent.getParcelableExtra<A>("EXTRA_A_OBJECT")

Make sure the key matches exactly between send and receive—typos here are a common culprit!

3. Complete A Class's Parcelable Implementation #

Your snippet cuts off at companion ...—you must implement the CREATOR for A to enable Parcelable deserialization. Here's the full companion object you need:

class A(var id: Int, var name: String, var bList: MutableList<B>) : Parcelable {
    override fun writeToParcel(p0: Parcel, p1: Int) {
        p0.writeInt(id)
        p0.writeString(name)
        p0.writeTypedList(bList)
    }

    override fun describeContents(): Int = 0

    constructor(parcel: Parcel) : this(
        parcel.readInt(),
        parcel.readString() ?: "", // Add default for non-null name
        parcel.createTypedArrayList(B.CREATOR) ?: mutableListOf() // Default empty list if null
    )

    // Missing CREATOR from your snippet—add this!
    companion object CREATOR : Parcelable.Creator<A> {
        override fun createFromParcel(parcel: Parcel): A = A(parcel)
        override fun newArray(size: Int): Array<A?> = arrayOfNulls(size)
    }
}

Notice we also added default values for name and bList to prevent nulls if the Parcel returns empty values.

4. Test with Simplified Objects #

If you're still getting null, test with a minimal A instance (e.g., empty bList or a single simple B object). If this works, the issue is likely in one of the B instances in your original list—check for invalid data or broken Parcelable logic in those objects.

内容的提问来源于stack exchange，提问作者Frank.Wang