Android studio Null指针异常：片段类中的结果不能为Null
我试图在我的多用途应用程序中实现一个计算器，但遇到了一个小问题。我已经在一个片段类中实现了计算器函数。每次我点击计算器碎片，整个应用程序都会崩溃 以下是代码，并从教程中获得了帮助 计算器片段Android studio Null指针异常：片段类中的结果不能为Null,android-studio,kotlin,android-fragments,calculator,Android Studio,Kotlin,Android Fragments,Calculator,我试图在我的多用途应用程序中实现一个计算器，但遇到了一个小问题。我已经在一个片段类中实现了计算器函数。每次我点击计算器碎片，整个应用程序都会崩溃 以下是代码，并从教程中获得了帮助 计算器片段 package com.khumomashapa.notes.fragments import android.os.Bundle import androidx.fragment.app.Fragment import android.view.LayoutInflater import and
package com.khumomashapa.notes.fragments
import android.os.Bundle
import androidx.fragment.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import com.khumomashapa.notes.R
import com.khumomashapa.notes.helper.OperationsHelper
import kotlinx.android.synthetic.main.fragment_calculator.*
class CalculatorFragment : Fragment() {
var digit_on_screen = StringBuilder(12)
var operation: Char = ' '
var leftHandSide: Double = 0.0
var rightHandSide: Double = 0.0
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
result_id.text = "0"
initializeButtons()
}
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_calculator, container, false)
}
private fun initializeButtons() {
functionalButtons()
operationalButtons()
numericalButtons()
}
/**
* This function initializes all of our numerical buttons from
* [0 - 9]
*/
private fun numericalButtons() {
one_btn.setOnClickListener {
appendToDigitOnScreen("1")
}
two_btn.setOnClickListener {
appendToDigitOnScreen("2")
}
three_btn.setOnClickListener {
appendToDigitOnScreen("3")
}
four_btn.setOnClickListener {
appendToDigitOnScreen("4")
}
five_btn.setOnClickListener {
appendToDigitOnScreen("5")
}
six_btn.setOnClickListener {
appendToDigitOnScreen("6")
}
seven_btn.setOnClickListener {
appendToDigitOnScreen("7")
}
eight_btn.setOnClickListener {
appendToDigitOnScreen("8")
}
nine_btn.setOnClickListener {
appendToDigitOnScreen("9")
}
zero_btn.setOnClickListener {
appendToDigitOnScreen("0")
}
dot_btn.setOnClickListener {
appendToDigitOnScreen(".")
}
}
/**
* Insert the button been clicked onto the screen so user can see
* inputs for the button clicked
*/
private fun appendToDigitOnScreen(digit: String) {
// Add each digit to our string builder
digit_on_screen.append(digit)
// display it on the screen of our mobile app
result_id.text = digit_on_screen.toString()
}
/**
* Initialize the operation keys in our calculator like the
* addition key, subtraction key and the likes
*/
private fun operationalButtons() {
addition_btn.setOnClickListener {
selectOperation('A')
}
subtract_btn.setOnClickListener {
selectOperation('S')
}
divide_btn.setOnClickListener {
selectOperation('D')
}
multipy_btn.setOnClickListener {
selectOperation('M')
}
}
/**
* Function to assign operational sign to our math calculations
*/
private fun selectOperation(c: Char) {
operation = c
leftHandSide = digit_on_screen.toString().toDouble()
digit_on_screen.clear()
result_id.text = "0"
}
/**
* Handles functional operations in out application like
* clear button, backspace button and the clear everything button
*/
private fun functionalButtons() {
clear_everything_btn.setOnClickListener {
digit_on_screen.clear()
result_id.text = "0"
}
clear_btn.setOnClickListener {
if (digit_on_screen.length <= 0) {
return@setOnClickListener
} else {
clearDigit()
}
}
backspace_btn.setOnClickListener {
if (digit_on_screen.length <= 0) {
return@setOnClickListener
} else {
clearDigit()
}
}
equal_btn.setOnClickListener {
performMathOperation()
}
}
/**
* This function performs our Math Operation which is then showed on the screen.
*/
private fun performMathOperation() {
rightHandSide = digit_on_screen.toString().toDouble()
when (operation) {
'A' -> {
val sum = OperationsHelper.add(leftHandSide, rightHandSide)
result_id.text = sum.toString()
digit_on_screen.clear()
digit_on_screen.append(sum)
}
'S' -> {
val subtract = OperationsHelper.subtract(leftHandSide, rightHandSide)
result_id.text = subtract.toString()
digit_on_screen.clear()
digit_on_screen.append(subtract)
}
'M' -> {
val multiply = OperationsHelper.multiply(leftHandSide, rightHandSide)
result_id.text = multiply.toString()
digit_on_screen.clear()
digit_on_screen.append(multiply)
}
'D' -> {
val divide = OperationsHelper.divide(leftHandSide, rightHandSide)
result_id.text = divide.toString()
digit_on_screen.clear()
digit_on_screen.append(divide)
}
}
}
/**
* This function remove the last digit on the screen.
*/
private fun clearDigit() {
val length = digit_on_screen.length
digit_on_screen.deleteCharAt(length - 1)
if (length <= 0) {
result_id.text = "0"
}else{
result_id.text = digit_on_screen.toString()
}
}
}
2021-03-28 11:25:35.688 25778-25778/? E/Zygote: isWhitelistProcess - Process is Whitelisted
2021-03-28 11:25:35.689 25778-25778/? E/Zygote: accessInfo : 1
2021-03-28 11:25:35.697 25778-25778/? I/momashapa.note: Late-enabling -Xcheck:jni
2021-03-28 11:25:35.716 25778-25778/? E/momashapa.note: Unknown bits set in runtime_flags: 0x8000
2021-03-28 11:25:35.724 25778-25778/? D/ActivityThread: setConscryptValidator
2021-03-28 11:25:35.724 25778-25778/? D/ActivityThread: setConscryptValidator - put
2021-03-28 11:25:36.122 25778-25778/com.khumomashapa.notes D/PhoneWindow: forceLight changed to true [] from com.android.internal.policy.PhoneWindow.updateForceLightNavigationBar:4274 com.android.internal.policy.DecorView.updateColorViews:1547 com.android.internal.policy.PhoneWindow.dispatchWindowAttributesChanged:3252 android.view.Window.setFlags:1153 com.android.internal.policy.PhoneWindow.generateLayout:2474
2021-03-28 11:25:36.123 25778-25778/com.khumomashapa.notes I/MultiWindowDecorSupport: [INFO] isPopOver = false
2021-03-28 11:25:36.123 25778-25778/com.khumomashapa.notes I/MultiWindowDecorSupport: updateCaptionType >> DecorView@ab7bf2e[], isFloating: false, isApplication: true, hasWindowDecorCaption: false, hasWindowControllerCallback: true
2021-03-28 11:25:36.123 25778-25778/com.khumomashapa.notes D/MultiWindowDecorSupport: setCaptionType = 0, DecorView = DecorView@ab7bf2e[]
2021-03-28 11:25:36.181 25778-25778/com.khumomashapa.notes W/momashapa.note: Accessing hidden method Landroid/view/View;->computeFitSystemWindows(Landroid/graphics/Rect;Landroid/graphics/Rect;)Z (greylist, reflection, allowed)
2021-03-28 11:25:36.182 25778-25778/com.khumomashapa.notes W/momashapa.note: Accessing hidden method Landroid/view/ViewGroup;->makeOptionalFitsSystemWindows()V (greylist, reflection, allowed)
2021-03-28 11:25:36.403 25778-25778/com.khumomashapa.notes I/ViewRootImpl@32ff46b[MainActivity]: setView = com.android.internal.policy.DecorView@ab7bf2e TM=true MM=false
2021-03-28 11:25:36.445 25778-25778/com.khumomashapa.notes I/ViewRootImpl@32ff46b[MainActivity]: Relayout returned: old=(0,0,1080,2400) new=(0,0,1080,2400) req=(1080,2400)0 dur=15 res=0x7 s={true 507110252544} ch=true
2021-03-28 11:25:36.445 25778-27904/com.khumomashapa.notes D/OpenGLRenderer: createReliableSurface : 0x758376d580(0x7612206000)
2021-03-28 11:25:36.446 25778-27904/com.khumomashapa.notes I/AdrenoGLES: QUALCOMM build : eb230ec, I000594fe7d
Build Date : 04/01/20
OpenGL ES Shader Compiler Version: EV031.27.05.03
Local Branch :
Remote Branch : refs/tags/AU_LINUX_ANDROID_LA.UM.8.9.R1.10.00.00.558.065
Remote Branch : NONE
Reconstruct Branch : NOTHING
2021-03-28 11:25:36.446 25778-27904/com.khumomashapa.notes I/AdrenoGLES: Build Config : S P 8.0.12 AArch64
2021-03-28 11:25:36.451 25778-27904/com.khumomashapa.notes I/AdrenoGLES: PFP: 0x016ee187, ME: 0x00000000
2021-03-28 11:25:36.481 25778-27904/com.khumomashapa.notes D/OpenGLRenderer: makeCurrent EglSurface : 0x0 -> 0x0
2021-03-28 11:25:36.485 25778-27904/com.khumomashapa.notes D/OpenGLRenderer: eglCreateWindowSurface : 0x757f3dce80
2021-03-28 11:25:36.522 25778-27904/com.khumomashapa.notes D/OpenGLRenderer: makeCurrent EglSurface : 0x0 -> 0x757f3dce80
2021-03-28 11:25:36.527 25778-27904/com.khumomashapa.notes W/Gralloc3: mapper 3.x is not supported
2021-03-28 11:25:36.625 25778-25778/com.khumomashapa.notes I/ViewRootImpl@32ff46b[MainActivity]: MSG_WINDOW_FOCUS_CHANGED 1 1
2021-03-28 11:25:36.626 25778-25778/com.khumomashapa.notes D/InputMethodManager: prepareNavigationBarInfo() DecorView@ab7bf2e[MainActivity]
2021-03-28 11:25:36.626 25778-25778/com.khumomashapa.notes D/InputMethodManager: getNavigationBarColor() -855310
2021-03-28 11:25:36.633 25778-25778/com.khumomashapa.notes D/InputMethodManager: prepareNavigationBarInfo() DecorView@ab7bf2e[MainActivity]
2021-03-28 11:25:36.633 25778-25778/com.khumomashapa.notes D/InputMethodManager: getNavigationBarColor() -855310
2021-03-28 11:25:36.633 25778-25778/com.khumomashapa.notes V/InputMethodManager: Starting input: tba=com.khumomashapa.notes ic=null mNaviBarColor -855310 mIsGetNaviBarColorSuccess true , NavVisible : true , NavTrans : false
2021-03-28 11:25:36.633 25778-25778/com.khumomashapa.notes D/InputMethodManager: startInputInner - Id : 0
2021-03-28 11:25:36.633 25778-25778/com.khumomashapa.notes I/InputMethodManager: startInputInner - mService.startInputOrWindowGainedFocus
2021-03-28 11:25:36.647 25778-25778/com.khumomashapa.notes I/ViewRootImpl@32ff46b[MainActivity]: MSG_RESIZED: frame=(0,0,1080,2400) ci=(0,91,0,126) vi=(0,91,0,126) or=1
2021-03-28 11:25:36.655 25778-25778/com.khumomashapa.notes D/InputMethodManager: prepareNavigationBarInfo() DecorView@ab7bf2e[MainActivity]
2021-03-28 11:25:36.655 25778-25778/com.khumomashapa.notes D/InputMethodManager: getNavigationBarColor() -855310
2021-03-28 11:25:36.655 25778-25778/com.khumomashapa.notes V/InputMethodManager: Starting input: tba=com.khumomashapa.notes ic=null mNaviBarColor -855310 mIsGetNaviBarColorSuccess true , NavVisible : true , NavTrans : false
2021-03-28 11:25:36.656 25778-25778/com.khumomashapa.notes D/InputMethodManager: startInputInner - Id : 0
2021-03-28 11:25:37.295 25778-25778/com.khumomashapa.notes I/ViewRootImpl@32ff46b[MainActivity]: ViewPostIme pointer 0
2021-03-28 11:25:37.334 25778-25778/com.khumomashapa.notes I/ViewRootImpl@32ff46b[MainActivity]: ViewPostIme pointer 1
2021-03-28 11:25:37.363 25778-25778/com.khumomashapa.notes D/AndroidRuntime: Shutting down VM
2021-03-28 11:25:37.364 25778-25778/com.khumomashapa.notes E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.khumomashapa.notes, PID: 25778
java.lang.NullPointerException: result_id must not be null
at com.khumomashapa.notes.fragments.CalculatorFragment.onCreate(CalculatorFragment.kt:22)
at androidx.fragment.app.Fragment.performCreate(Fragment.java:2586)
at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:838)
at androidx.fragment.app.FragmentTransition.addToFirstInLastOut(FragmentTransition.java:1197)
at androidx.fragment.app.FragmentTransition.calculateFragments(FragmentTransition.java:1080)
at androidx.fragment.app.FragmentTransition.startTransitions(FragmentTransition.java:119)
at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1866)
at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1824)
at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManagerImpl.java:1727)
at androidx.fragment.app.FragmentManagerImpl$2.run(FragmentManagerImpl.java:150)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:237)
at android.app.ActivityThread.main(ActivityThread.java:8167)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:496)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1100)
2021-03-28 11:25:37.385 25778-25778/com.khumomashapa.notes I/Process: Sending signal. PID: 25778 SIG: 9
对于片段，
onCreateonViewCreated您可以在中查看有关片段生命周期的更多详细信息。对于片段，在创建视图之前会调用
onCreateonViewCreated您可以在中更详细地了解片段生命周期。哦，我明白了。我将尝试一下onViewCreated方法，看看它对我的效果如何。哦，我理解。我将尝试一下onViewCreated方法，看看它对我的效果如何