diff --git a/libs/input/VelocityTracker.cpp b/libs/input/VelocityTracker.cpp
index 75ad71c..b174fa8 100644
--- a/libs/input/VelocityTracker.cpp
+++ b/libs/input/VelocityTracker.cpp
@@ -27,10 +27,10 @@
 #include <limits.h>
 #include <math.h>
 
+#include <android-base/stringprintf.h>
 #include <cutils/properties.h>
 #include <input/VelocityTracker.h>
 #include <utils/BitSet.h>
-#include <utils/String8.h>
 #include <utils/Timers.h>
 
 namespace android {
@@ -63,36 +63,36 @@
 }
 
 #if DEBUG_STRATEGY || DEBUG_VELOCITY
-static String8 vectorToString(const float* a, uint32_t m) {
-    String8 str;
-    str.append("[");
+static std::string vectorToString(const float* a, uint32_t m) {
+    std::string str;
+    str += "[";
     for (size_t i = 0; i < m; i++) {
         if (i) {
-            str.append(",");
+            str += ",";
         }
-        str.appendFormat(" %f", *(a++));
+        str += android::base::StringPrintf(" %f", *(a++));
     }
-    str.append(" ]");
+    str += " ]";
     return str;
 }
 
-static String8 matrixToString(const float* a, uint32_t m, uint32_t n, bool rowMajor) {
-    String8 str;
-    str.append("[");
+static std::string matrixToString(const float* a, uint32_t m, uint32_t n, bool rowMajor) {
+    std::string str;
+    str = "[";
     for (size_t i = 0; i < m; i++) {
         if (i) {
-            str.append(",");
+            str += ",";
         }
-        str.append(" [");
+        str += " [";
         for (size_t j = 0; j < n; j++) {
             if (j) {
-                str.append(",");
+                str += ",";
             }
-            str.appendFormat(" %f", a[rowMajor ? i * n + j : j * m + i]);
+            str += android::base::StringPrintf(" %f", a[rowMajor ? i * n + j : j * m + i]);
         }
-        str.append(" ]");
+        str += " ]";
     }
-    str.append(" ]");
+    str += " ]";
     return str;
 }
 #endif
@@ -255,8 +255,8 @@
                 "estimator (degree=%d, xCoeff=%s, yCoeff=%s, confidence=%f)",
                 id, positions[index].x, positions[index].y,
                 int(estimator.degree),
-                vectorToString(estimator.xCoeff, estimator.degree + 1).string(),
-                vectorToString(estimator.yCoeff, estimator.degree + 1).string(),
+                vectorToString(estimator.xCoeff, estimator.degree + 1).c_str(),
+                vectorToString(estimator.yCoeff, estimator.degree + 1).c_str(),
                 estimator.confidence);
     }
 #endif
@@ -442,8 +442,8 @@
         const float* w, uint32_t m, uint32_t n, float* outB, float* outDet) {
 #if DEBUG_STRATEGY
     ALOGD("solveLeastSquares: m=%d, n=%d, x=%s, y=%s, w=%s", int(m), int(n),
-            vectorToString(x, m).string(), vectorToString(y, m).string(),
-            vectorToString(w, m).string());
+            vectorToString(x, m).c_str(), vectorToString(y, m).c_str(),
+            vectorToString(w, m).c_str());
 #endif
 
     // Expand the X vector to a matrix A, pre-multiplied by the weights.
@@ -455,7 +455,7 @@
         }
     }
 #if DEBUG_STRATEGY
-    ALOGD("  - a=%s", matrixToString(&a[0][0], m, n, false /*rowMajor*/).string());
+    ALOGD("  - a=%s", matrixToString(&a[0][0], m, n, false /*rowMajor*/).c_str());
 #endif
 
     // Apply the Gram-Schmidt process to A to obtain its QR decomposition.
@@ -490,8 +490,8 @@
         }
     }
 #if DEBUG_STRATEGY
-    ALOGD("  - q=%s", matrixToString(&q[0][0], m, n, false /*rowMajor*/).string());
-    ALOGD("  - r=%s", matrixToString(&r[0][0], n, n, true /*rowMajor*/).string());
+    ALOGD("  - q=%s", matrixToString(&q[0][0], m, n, false /*rowMajor*/).c_str());
+    ALOGD("  - r=%s", matrixToString(&r[0][0], n, n, true /*rowMajor*/).c_str());
 
     // calculate QR, if we factored A correctly then QR should equal A
     float qr[n][m];
@@ -503,7 +503,7 @@
             }
         }
     }
-    ALOGD("  - qr=%s", matrixToString(&qr[0][0], m, n, false /*rowMajor*/).string());
+    ALOGD("  - qr=%s", matrixToString(&qr[0][0], m, n, false /*rowMajor*/).c_str());
 #endif
 
     // Solve R B = Qt W Y to find B.  This is easy because R is upper triangular.
@@ -521,7 +521,7 @@
         outB[i] /= r[i][i];
     }
 #if DEBUG_STRATEGY
-    ALOGD("  - b=%s", vectorToString(outB, n).string());
+    ALOGD("  - b=%s", vectorToString(outB, n).c_str());
 #endif
 
     // Calculate the coefficient of determination as 1 - (SSerr / SStot) where
@@ -607,8 +607,8 @@
 #if DEBUG_STRATEGY
             ALOGD("estimate: degree=%d, xCoeff=%s, yCoeff=%s, confidence=%f",
                     int(outEstimator->degree),
-                    vectorToString(outEstimator->xCoeff, n).string(),
-                    vectorToString(outEstimator->yCoeff, n).string(),
+                    vectorToString(outEstimator->xCoeff, n).c_str(),
+                    vectorToString(outEstimator->yCoeff, n).c_str(),
                     outEstimator->confidence);
 #endif
             return true;