我正在创建一个Android应用程序（Android Studio），其中主要活动是使用ViewPager的活动。 每个页面显示118个元素之一的一些信息，第一个元素除外，它显示了所有元素的列表。
我做了一个Element枚举，其中存储了所有元素。他们的名字也存储在那里，我想从字符串资源（res/values/strings.xml）获取。由于Element是一个独立的enum，因此它没有从中获取字符串的上下文。我通过在主要活动（public static Context context）中设置ElementPagerActivity.java来解决此问题。正如您在下面的代码中看到的那样，我会在其构造函数中为每个context调用Element。
但是，当我执行（nexus 5）时，Logcat将显示以下错误：
java.lang.ExceptionInInitializerError
...
Caused by: java.lang.NullPointerException: Attempt to invoke a virtual method on a null object reference
at com.teinproductions.tein.molu.Element.<clinit>(Element.java:8)
at com.teinproductions.tein.molu.ElementPagerActivity$1.getCount(ElementPagerActivity.java:40)
at android.support.v4.view.ViewPager.setAdapter(ViewPager.java:439)
at com.teinproductions.tein.molu.ElementPagerActivity.onCreate(ElementPagerActivity.java:28)
...
Element.java:8指的是以下行：
HYDROGEN(ElementPagerActivity.context.getResources().getString(R.string.hydrogen),"H",1.0079,1),
这是Element.java枚举中元素的第一个声明。
此错误是否表明ElementPagerActivity引用了null？我该如何解决这个错误？
这是我的代码（该项目的所有代码都在github上：https://github.com/teinvdlugt/molu）
Element.java（是的，有许多要素）：
package com.teinproductions.tein.molu;
import android.os.Bundle;
public enum Element {
HYDROGEN(ElementPagerActivity.context.getResources().getString(R.string.hydrogen),"H",1.0079,1),
HELIUM(ElementPagerActivity.context.getResources().getString(R.string.helium),"He",4.0026,2),
LITHIUM(ElementPagerActivity.context.getResources().getString(R.string.lithium),"Li",6.941,3),
BERYLLIUM(ElementPagerActivity.context.getResources().getString(R.string.beryllium),"Be",9.0122,4),
BORON(ElementPagerActivity.context.getResources().getString(R.string.boron),"B",10.811,5),
CARBON(ElementPagerActivity.context.getResources().getString(R.string.carbon),"C",12.0107,6),
NITROGEN(ElementPagerActivity.context.getResources().getString(R.string.nitrogen),"N",14.0067,7),
OXYGEN(ElementPagerActivity.context.getResources().getString(R.string.oxygen),"O",15.9994,8),
FLUORINE(ElementPagerActivity.context.getResources().getString(R.string.fluorine),"F",18.9984,9),
NEON(ElementPagerActivity.context.getResources().getString(R.string.neon),"Ne",20.1797,10),
SODIUM(ElementPagerActivity.context.getResources().getString(R.string.sodium),"Na",22.9897,11),
MAGNESIUM(ElementPagerActivity.context.getResources().getString(R.string.magnesium),"Mg",24.305,12),
ALUMINUM(ElementPagerActivity.context.getResources().getString(R.string.aluminum),"Al",26.9815,13),
SILICON(ElementPagerActivity.context.getResources().getString(R.string.silicon),"Si",28.0855,14),
PHOSPHORUS(ElementPagerActivity.context.getResources().getString(R.string.phosphorus),"P",30.9738,15),
SULFUR(ElementPagerActivity.context.getResources().getString(R.string.sulfur),"S",32.065,16),
CHLORINE(ElementPagerActivity.context.getResources().getString(R.string.chlorine),"Cl",35.453,17),
ARGON(ElementPagerActivity.context.getResources().getString(R.string.argon),"Ar",39.948,18),
POTASSIUM(ElementPagerActivity.context.getResources().getString(R.string.potassium),"K",39.0983,19),
CALCIUM(ElementPagerActivity.context.getResources().getString(R.string.calcium),"Ca",40.078,20),
SCANDIUM(ElementPagerActivity.context.getResources().getString(R.string.scandium),"Sc",44.9559,21),
TITANIUM(ElementPagerActivity.context.getResources().getString(R.string.titanium),"Ti",47.867,22),
VANADIUM(ElementPagerActivity.context.getResources().getString(R.string.vanadium),"V",50.9415,23),
CHROMIUM(ElementPagerActivity.context.getResources().getString(R.string.chromium),"Cr",51.9961,24),
MANGANESE(ElementPagerActivity.context.getResources().getString(R.string.manganese),"Mn",54.938,24),
IRON(ElementPagerActivity.context.getResources().getString(R.string.iron),"Fe",55.845,26),
COBALT(ElementPagerActivity.context.getResources().getString(R.string.cobalt),"Co",58.6934,27),
NICKEL(ElementPagerActivity.context.getResources().getString(R.string.nickel),"Ni",58.6934,28),
COPPER(ElementPagerActivity.context.getResources().getString(R.string.copper),"Cu",63.546,29),
ZINC(ElementPagerActivity.context.getResources().getString(R.string.zinc),"Zn",65.39,30),
GALLIUM(ElementPagerActivity.context.getResources().getString(R.string.gallium),"Ga",69.723,31),
GERMANIUM(ElementPagerActivity.context.getResources().getString(R.string.germanium),"Ge",72.64,32),
ARSENIC(ElementPagerActivity.context.getResources().getString(R.string.arsenic),"As",74.9216,33),
SELENIUM(ElementPagerActivity.context.getResources().getString(R.string.selenium),"Se",78.96,34),
BROMINE(ElementPagerActivity.context.getResources().getString(R.string.bromine),"Br",79.904,35),
KRYPTON(ElementPagerActivity.context.getResources().getString(R.string.krypton),"Kr",83.8,36),
RUBIDIUM(ElementPagerActivity.context.getResources().getString(R.string.rubidium),"Rb",85.4678,37),
STRONTIUM(ElementPagerActivity.context.getResources().getString(R.string.strontium),"Sr",87.62,38),
YTTRIUM(ElementPagerActivity.context.getResources().getString(R.string.yttrium),"Y",88.9095,39),
ZIRCONIUM(ElementPagerActivity.context.getResources().getString(R.string.zirconium),"Zr",91.224,40),
NIOBIUM(ElementPagerActivity.context.getResources().getString(R.string.niobium),"Nb",92.9064,41),
MOLYBDENUM(ElementPagerActivity.context.getResources().getString(R.string.molybdenum),"Mo",95.94,42),
TECHNETIUM(ElementPagerActivity.context.getResources().getString(R.string.technetium),"Tc",98.0,43),
RUTHENIUM(ElementPagerActivity.context.getResources().getString(R.string.ruthenium),"Ru",101.07,44),
RHODIUM(ElementPagerActivity.context.getResources().getString(R.string.rhodium),"Rh",102.9055,45),
PALLADIUM(ElementPagerActivity.context.getResources().getString(R.string.palladium),"Pd",106.42,46),
SILVER(ElementPagerActivity.context.getResources().getString(R.string.silver),"Ag",107.8682,47),
CADMIUM(ElementPagerActivity.context.getResources().getString(R.string.cadmium),"Cd",112.411,48),
INDIUM(ElementPagerActivity.context.getResources().getString(R.string.indium),"In",114.818,49),
TIN(ElementPagerActivity.context.getResources().getString(R.string.tin),"Sn",118.71,50),
ANTIMONY(ElementPagerActivity.context.getResources().getString(R.string.antimony),"Sb",121.76,51),
TELLURIUM(ElementPagerActivity.context.getResources().getString(R.string.tellurium),"Te",127.6,52),
IODINE(ElementPagerActivity.context.getResources().getString(R.string.iodine),"I",126.9045,53),
XENON(ElementPagerActivity.context.getResources().getString(R.string.xenon),"Xe",131.293,54),
CESIUM(ElementPagerActivity.context.getResources().getString(R.string.cesium),"Cs",132.9055,55),
BARIUM(ElementPagerActivity.context.getResources().getString(R.string.barium),"Ba",137.327,56),
LANTHANUM(ElementPagerActivity.context.getResources().getString(R.string.lanthanum),"La",138.9055,57),
CERIUM(ElementPagerActivity.context.getResources().getString(R.string.cerium),"Ce",140.116,58),
PRASEODYMIUM(ElementPagerActivity.context.getResources().getString(R.string.praseodymium),"Pr",140.9077,59),
NEODYMIUM(ElementPagerActivity.context.getResources().getString(R.string.neodymium),"Nd",144.24,60),
PROMETHIUM(ElementPagerActivity.context.getResources().getString(R.string.promethium),"Pm",(double)145,61),
SAMARIUM(ElementPagerActivity.context.getResources().getString(R.string.samarium),"Sm",150.36,62),
EUROPIUM(ElementPagerActivity.context.getResources().getString(R.string.europium),"Eu",151.964,63),
GADOLINIUM(ElementPagerActivity.context.getResources().getString(R.string.gadolinium),"Gd",157.25,64),
TERBIUM(ElementPagerActivity.context.getResources().getString(R.string.terbium),"Tb",158.9253,65),
DYSPROSIUM(ElementPagerActivity.context.getResources().getString(R.string.dysprosium),"Dy",162.5,66),
HOLMIUM(ElementPagerActivity.context.getResources().getString(R.string.holmium),"Ho",164.9303,67),
ERBIUM(ElementPagerActivity.context.getResources().getString(R.string.erbium),"Er",167.259,68),
THULIUM(ElementPagerActivity.context.getResources().getString(R.string.thulium),"Tm",168.9342,69),
YTTERBIUM(ElementPagerActivity.context.getResources().getString(R.string.ytterbium),"Yb",173.04,70),
LUTETIUM(ElementPagerActivity.context.getResources().getString(R.string.lutetium),"Lu",174.967,71),
HAFNIUM(ElementPagerActivity.context.getResources().getString(R.string.hafnium),"Hf",178.49,72),
TANTALUM(ElementPagerActivity.context.getResources().getString(R.string.tantalum),"Ta",180.9479,73),
TUNGSTEN(ElementPagerActivity.context.getResources().getString(R.string.tungsten),"W",183.84,74),
RHENIUM(ElementPagerActivity.context.getResources().getString(R.string.rhenium),"Re",186.207,75),
OSMIUM(ElementPagerActivity.context.getResources().getString(R.string.osmium),"Os",190.23,76),
IRIDIUM(ElementPagerActivity.context.getResources().getString(R.string.iridium),"Ir",192.217,77),
PLATINUM(ElementPagerActivity.context.getResources().getString(R.string.platinum),"Pt",195.078,78),
GOLD(ElementPagerActivity.context.getResources().getString(R.string.gold),"Au",196.9665,79),
MERCURY(ElementPagerActivity.context.getResources().getString(R.string.mercury),"Hg",200.59,80),
THALLIUM(ElementPagerActivity.context.getResources().getString(R.string.thallium),"Tl",204.3833,81),
LEAD(ElementPagerActivity.context.getResources().getString(R.string.lead),"Pb",207.2,82),
BISMUTH(ElementPagerActivity.context.getResources().getString(R.string.bismuth),"Bi",208.9804,83),
POLONIUM(ElementPagerActivity.context.getResources().getString(R.string.polonium), "Po", 209.0, 84),
ASTATINE(ElementPagerActivity.context.getResources().getString(R.string.astatine), "At", 210.0, 85),
RADON(ElementPagerActivity.context.getResources().getString(R.string.radon),"Rn",222.0,86),
FRANCIUM(ElementPagerActivity.context.getResources().getString(R.string.francium),"Fr",223.0,87),
RADIUM(ElementPagerActivity.context.getResources().getString(R.string.radium),"Ra",226.0,88),
ACTINIUM(ElementPagerActivity.context.getResources().getString(R.string.actinium),"Ac",227.0,89),
THORIUM(ElementPagerActivity.context.getResources().getString(R.string.thorium),"Th",232.0381,90),
PROTACTINIUM(ElementPagerActivity.context.getResources().getString(R.string.protactinium),"Pa",231.0359,91),
URANIUM(ElementPagerActivity.context.getResources().getString(R.string.uranium),"U",238.0289,92),
NEPTUNIUM(ElementPagerActivity.context.getResources().getString(R.string.neptunium),"Np",237.0,93),
PLUTONIUM(ElementPagerActivity.context.getResources().getString(R.string.plutonium),"Pu",244.0,94),
AMERICIUM(ElementPagerActivity.context.getResources().getString(R.string.americium),"Am",243.0,95),
CURIUM(ElementPagerActivity.context.getResources().getString(R.string.curium),"Cm",247.0,96),
BERKELIUM(ElementPagerActivity.context.getResources().getString(R.string.berkelium),"Bk",247.0,97),
CALIFORNIUM(ElementPagerActivity.context.getResources().getString(R.string.californium),"Cf",251.0,98),
EINSTEINIUM(ElementPagerActivity.context.getResources().getString(R.string.einsteinium),"Es",252.0,99),
FERMIUM(ElementPagerActivity.context.getResources().getString(R.string.fermium),"Fm",257.0,100),
MENDELEVIUM(ElementPagerActivity.context.getResources().getString(R.string.mendelevium),"Md",258.0,101),
NOBELIUM(ElementPagerActivity.context.getResources().getString(R.string.nobelium),"No",259.0,102),
LAWRENCIUM(ElementPagerActivity.context.getResources().getString(R.string.lawrencium),"Lr",262.0,103),
RUTHERFORDIUM(ElementPagerActivity.context.getResources().getString(R.string.rutherfordium),"Rf",261.0,104),
DUBNIUM(ElementPagerActivity.context.getResources().getString(R.string.dubnium),"Db",262.0,105),
SEABORGIUM(ElementPagerActivity.context.getResources().getString(R.string.seaborgium),"Sg",266.0,106),
BOHRIUM(ElementPagerActivity.context.getResources().getString(R.string.bohrium),"Bh",264.0,107),
HASSIUM(ElementPagerActivity.context.getResources().getString(R.string.hassium),"Hs",277.0,108),
MEITNERIUM(ElementPagerActivity.context.getResources().getString(R.string.meitnerium),"Mt",269.0,109),
DARMSTADTIUM(ElementPagerActivity.context.getResources().getString(R.string.darmstadtium),"Ds",281.0,110),
ROENTGENIUM(ElementPagerActivity.context.getResources().getString(R.string.roentgenium),"Rg",281.0,111),
COPERNICIUM(ElementPagerActivity.context.getResources().getString(R.string.copernicium),"Cn",285.0,112),
UNUNTRIUM(ElementPagerActivity.context.getResources().getString(R.string.ununtrium),"Uut",286.0,113),
FLEROVIUM(ElementPagerActivity.context.getResources().getString(R.string.flerovium),"Fl",289.0,114),
UNUNPENTIUM(ElementPagerActivity.context.getResources().getString(R.string.ununpentium),"Uup",289.0,115),
LIVERMORIUM(ElementPagerActivity.context.getResources().getString(R.string.livermorium),"Lv",293.0,116),
UNUNSEPTIUM(ElementPagerActivity.context.getResources().getString(R.string.ununseptium),"Uus",294.0,117),
UNUNOCTIUM(ElementPagerActivity.context.getResources().getString(R.string.ununoctium),"Uuo",294.0,118);
private String name,abbreviation;
private Double mass;
private int atomicNumber;
public static final Double nA = 602214000000000000000000.0;
public static final String ELEMENT = "ELEMENT_TO_PASS_TO_FRAGMENT_ARGUMENTS";
Element(String name, String abbreviation, Double mass, Integer atomicNumber) {
this.name = name;
this.abbreviation = abbreviation;
this.mass = mass;
this.atomicNumber = atomicNumber;
}
public Double calculateGramWhenMolGiven(Double givenMol){
return givenMol * this.mass;
}
public Double calculateParticlesWhenMolGiven(Double givenMol){
return givenMol * nA;
}
public Double calculateMolWhenGramGiven(Double givenGram){
return givenGram / this.mass;
}
public Double calculateMolWhenParticlesGiven(Integer givenParticles){
return givenParticles / nA;
}
public static Element findElementByAbbreviation(String abbreviation){
for(Element currentElement : Element.values()){
if(abbreviation.equalsIgnoreCase(currentElement.getAbbreviation())){
return currentElement;
}
}
return null;
}
public static Element findElementByName(String name){
for(Element currentElement : Element.values()){
if(currentElement.getName().equalsIgnoreCase(name)) {
return currentElement;
}
}
return null;
}
public static Element findElementByAbbreviationOrName(String nameOrAbbreviation){
Element foundElement;
foundElement = Element.findElementByAbbreviation(nameOrAbbreviation);
if(foundElement == null){
foundElement = Element.findElementByName(nameOrAbbreviation);
}
return foundElement;
}
public ElementFragment toFragment(){
ElementFragment elementFragment = new ElementFragment();
Bundle args = new Bundle();
args.putSerializable(ELEMENT, this);
elementFragment.setArguments(args);
return elementFragment;
}
public String getName() {
return name;
}
public String getAbbreviation() {
return abbreviation;
}
public Double getMass() {
return mass;
}
public Integer getAtomicNumber() {
return atomicNumber;
}
}
ElementPagerActivity.java：
package com.teinproductions.tein.molu;
import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentStatePagerAdapter;
import android.support.v4.view.PagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.ActionBarActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
public class ElementPagerActivity extends ActionBarActivity {
ViewPager viewPager;
// To allow the Element enumeration to get strings from the resources:
public static Context context;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
context = getApplicationContext();
// This class doesn't use a layout resources file.
viewPager = new ViewPager(this);
setContentView(viewPager);
viewPager.setAdapter(new FragmentStatePagerAdapter(getSupportFragmentManager()) {
@Override
public Fragment getItem(int position) {
if(position == 0){
return new ElementListFragment();
}
return Element.values()[position + 1].toFragment();
}
@Override
public int getCount() {
return Element.values().length + 1;
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.menu_element_pager_activity, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if(id == R.id.action_bar_element_pager_activity_list){
// scrolls the viewpager back to the list of elements,
// which is at index number 0
viewPager.setCurrentItem(0);
}
return super.onOptionsItemSelected(item);
}
}
我已经找到了关于此错误的更多问题，但我找不到解决了我的问题。
感谢您的帮助！ 如果您需要项目中的更多代码，请说出来。
谢谢，
来自荷兰的Tein