H07000223/FlycoTabLayout · error · IllegalStateException

Titles length must be the same as the page count !

Error message

Titles length must be the same as the page count !

What it means

SlidingTabLayout.setViewPager(ViewPager, String[]) also verifies that titles.length equals vp.getAdapter().getCount(); a mismatch throws IllegalStateException "Titles length must be the same as the page count !". Tab count and page count must stay 1:1 or clicks/scrolls will map to the wrong pages.

Solutions

  1. Update the titles array so its length matches adapter.getCount(), then re-call setViewPager.
  2. Better: implement getPageTitle() in the PagerAdapter and use the 1-arg setViewPager so titles live in one place.
  3. On adapter data changes, rebuild titles and rebind the tab layout in the same code path.
  4. Add an assertion/unit test comparing titles.length to adapter.getCount() before binding.

Example fix

// before
String[] titles = {"A", "B"}; // adapter has 3 pages
slidingTabLayout.setViewPager(viewPager, titles);
// after
String[] titles = {"A", "B", "C"}; // matches adapter.getCount()
slidingTabLayout.setViewPager(viewPager, titles);
Defensive patterns

Strategy: validation

Validate before calling

if (vp.getAdapter() != null && titles != null && titles.length == vp.getAdapter().getCount()) {
    slidingTabLayout.setViewPager(vp, titles);
}

Type guard

boolean titlesMatchPages(ViewPager vp, String[] titles) {
    return vp.getAdapter() != null && titles != null
        && titles.length == vp.getAdapter().getCount();
}

Try / catch

try {
    slidingTabLayout.setViewPager(vp, titles);
} catch (IllegalStateException e) {
    Log.e("SlidingTabLayout", "titles/page count mismatch", e);
}

Prevention

When it happens

Trigger: Supplying more or fewer titles than the adapter's page count, e.g. adding a fragment to the adapter without updating the titles array, or filtering one list but not the other.

Common situations: Dynamic adapters whose count changes after initial setup; hardcoded titles arrays that drifted from the fragment list; merging branches where one side added a page only.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of H07000223/FlycoTabLayout@528fcffe88 (2026-09-08). Data as JSON: /api/errors/411d8629a333e4db. Report an issue: GitHub.

Appendix: source

Thrown at FlycoTabLayout_Lib/src/main/java/com/flyco/tablayout/SlidingTabLayout.java:196

        this.mViewPager = vp;

        this.mViewPager.removeOnPageChangeListener(this);
        this.mViewPager.addOnPageChangeListener(this);
        notifyDataSetChanged();
    }

    /** 关联ViewPager,用于不想在ViewPager适配器中设置titles数据的情况 */
    public void setViewPager(ViewPager vp, String[] titles) {
        if (vp == null || vp.getAdapter() == null) {
            throw new IllegalStateException("ViewPager or ViewPager adapter can not be NULL !");
        }

        if (titles == null || titles.length == 0) {
            throw new IllegalStateException("Titles can not be EMPTY !");
        }

        if (titles.length != vp.getAdapter().getCount()) {
            throw new IllegalStateException("Titles length must be the same as the page count !");
        }

        this.mViewPager = vp;
        mTitles = new ArrayList<>();
        Collections.addAll(mTitles, titles);

        this.mViewPager.removeOnPageChangeListener(this);
        this.mViewPager.addOnPageChangeListener(this);
        notifyDataSetChanged();
    }

    /** 关联ViewPager,用于连适配器都不想自己实例化的情况 */
    public void setViewPager(ViewPager vp, String[] titles, FragmentActivity fa, ArrayList<Fragment> fragments) {
        if (vp == null) {
            throw new IllegalStateException("ViewPager can not be NULL !");
        }

        if (titles == null || titles.length == 0) {

View on GitHub (pinned to 528fcffe88)