Difference between KeyManager and TrustManager?

Question

I'm reading up on the difference between keymanager and trustmanager but still quite not sure why only using keymanager works for my code.

I'm writing some client codes to upload images using HTTPS in android to the SSL server, using a self-signed certificate provided. Here's a snippet of the codes:

CertificateFactory cf = CertificateFactory.getInstance("X.509");
// load ca                  
InputStream caInput = getActivity().getResources().getAssets().open("qa_upld.cer");
Certificate ca;
try {
    ca = cf.generateCertificate(caInput);
    System.out.println("ca = " + ((X509Certificate) ca).getSubjectDN());
} finally {
    caInput.close();
}
// Create a KeyStore 
KeyStore keyStore = KeyStore.getInstance(KeyStore.getDefaultType());
keyStore.load(null, null);
keyStore.setCertificateEntry("ca", ca);

KeyManagerFactory kmf = KeyManagerFactory.getInstance("X509");
kmf.init(keyStore, null);
KeyManager[] keyManagers = kmf.getKeyManagers(); 

// Create a TrustManager that trusts the CAs in our KeyStore
//TrustManagerFactory tmf = TrustManagerFactory.getInstance("X509");
//tmf.init(keyStore); 
// Using trustmanager not working for some reason..

// Create an SSLContext that uses our TrustManager
    // Out of 3 possibilities, (keymanager,trustmanager),(null,trustmanager) 
    // and (keymanager,null) only the following works
SSLContext context = SSLContext.getInstance("TLS");
context.init(keyManagers, null, null);

My question is why is using trustmanager not working in this case? I ran into "trust anchor not found" while testing the case, which confused me because I thought I already installed the trustmanager? Is it because I'm using a self-signed certificate, not a public one?

Switching to keymanager seems to solve the issue.