android - How to save data and re-open the last used Activity -

i've made majority of game's mechanics, need able to:

1. save data of current activity , retrieve when coming (i'd appreciate example of sharedpreferences if that's need)

2. open back same activity left and in same time when left it.

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just clearer: i want not restart main activity each time app gets closed or killed.

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edit:

alright, i've used google article in order save activity , recreate later.

the code in 1 of activities following:

oncreate()

@override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_player_selection);      // check whether we're recreating destroyed instance     if (savedinstancestate != null) {         // restore value of members saved state         village = savedinstancestate.getstring(village);         seekbarprogress = savedinstancestate.getint(progress);     } else {         // initialize members default values new instance         initializevariables();         bundle bundle = getintent().getextras();         village = bundle.getstring(village);     }         [...] // other code skipped     } 

onsaveinstancestate()

@override public void onsaveinstancestate(bundle savedinstancestate) {     // save user's current game state     savedinstancestate.putstring(village, village);     savedinstancestate.putint(progress, seekbarprogress);     // call superclass can save view hierarchy state     super.onsaveinstancestate(savedinstancestate); } 

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thus, i've created dispatcher activity (which main) when app killed , launched back, knows last opened activity launch. when try that, , should open (i.e.) playerselection activity,

java.lang.nullpointerexception

as runs part of code

else { // initialize members default values new instance initializevariables(); bundle bundle = getintent().getextras(); village = bundle.getstring(village); }

instead of previous if statement runs else.

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dispatcher

this dispatcher activity filters activity should launched:

public class dispatcher extends activity {  @override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);      class<?> activityclass;      try {         sharedpreferences prefs = getsharedpreferences("x", mode_private);         activityclass = class.forname(                 prefs.getstring("lastactivity", mainactivity.class.getname()));     } catch(classnotfoundexception ex) {         activityclass = mainactivity.class;     }      startactivity(new intent(this, activityclass));     } } 

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this send dispatcher activity:

@override protected void onpause() {     super.onpause();      sharedpreferences prefs = getsharedpreferences("x", mode_private);     sharedpreferences.editor editor = prefs.edit();     editor.putstring("lastactivity", getclass().getname());     editor.commit();     } } 

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question

why happen? can fix this?

why happen?

the bundle created in onsaveinstancestate kept short-term restarts, e.g. when activity recreated due orientation change. if activity stops bundle not kept, , can't delivered oncreate.

what can fix this?

instead of using instancestate model, save activity state in sharedpreferences (similar lastactivity save). should save state in onstop or onpause , restore in onstart or onresume respectively. in cases former enough , latter produces unnecessary overhead.

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@override protected void onstart() {     super.onstart();     string saved = sharedpreferences.getstring("mykey", null);     mypojo pojo;     if(saved == null){         pojo = defaultvalue;     }else {         pojo = new gson().fromjson(saved, mypojo.class);     } }  @override protected void onstop() {     sharedpreferences.edit().putstring("mykey", new gson().tojson(pojo)).apply();     super.onstop(); }